The game is best thought of as a means to read a story. To that end, until you are approaching the high level (all stats at or near cap and so forth), the best sources of Echos are the content you're seeing. The only exception to this rule is that when you see content related to A box of secrets, play that content as that leads to the "gold standard" Echo grind.
(The short version, for those looking for a TL/DR: Your best option (in the long run, at least) is to curate your deck carefully, play good cards and discard crappy ones, attend parties, and use your relatively small number of remaining actions to play the Collection of Curiosities branch of the Dramatic Tension carousel and trade the resulting Collections to tomb colonists. Second best is probably grinding Port Carnelian if you know what you're doing.)
Your best methods for echo grinding are the following:
- Fate-locked selling your soul (28.15 epa, increasing amounts of Fate required)
- Fate-locked expeditions (7 Fate for 5.3 epa [under correct conditions])
- Tanah-Chook, unlocked by the "All Things Must End" Exceptional story (1.8 epa)
- Fate-locked selling your soul (1.8 epa, easily locked out of)
- Fate-locked Soul Trade (1.75 epa)
The following can be done only with skill-checks.
- The Court of the Wakeful Eye lets you buy Winsome Dispossessed Orphans, use them to get tribute, and turn that tribute into more echoes than you spent (2.22 epa minus the actions of zailling). The more orphans you invest in to begin, the closer this comes to 2.22 epa.
- Collections of Curiosities via the Tomb-Colonies faction card (1.8 epa, 65e/36a [1.81 epa])
- While this calculation true in a very narrow sense, it's not true when taking account of opportunity costs. Two of those 36 actions are generating 1 x Favours: Tomb-Colonies, and then cashing it in on Bandages and Dust: The Tomb-Colonies. If not grinding in this fashion, both of those card-plays would likely still occur, but the latter would be used to generate another 1 x Favours: Tomb-Colonies, and two actions would then be spent to cash those two Favours in for 8.4 echoes worth of Primordial Shrieks. Taking that into account, the true net EPA is (65 - 8.4)/(36 - 4) = 1.77 EPA.
- Investigations in the university and cashing in the Flit (~1.78 epa, easily locked out of)
- Boxfuls of Intrigue with the correct choices (1.64 epa, 23e/14a)
The following have a range in pay-outs and should only be considered in large batches for an overall epa.
- Polythreme with a 5 card lodgings (2.1 epa on a perfect run, averages 1.71 - 1.76 epa depending on luck odds and excluding the cost of getting to/from Polythreme and acquiring an extra card for the grind)
- Tale of the Fidgeting Writer (~1.92 epa without grinding items, ~1.57 with grinding [batches of 1000+ ToT recommended])
- Heists in the Flit (1.94 to 1.998 epa, depending on risks taken. Read the entire comment section for details.) [Searing Enigma rare success chance lowered, needs recalculating, still somewhat viable?]
- Port Carnelian (1.9+ epa)
- 2.1+ EPA with optimal (within sane limits) strategy, which I really need to finish running the exact numbers on and write up.
These grinds worked for some people at points in the games past but currently are not believed to be profitable.
- Hunter's Keep Searing Enigma rare success (Used to be 1.85 epa, but has since been decreased, might now be ~1.5 epa)
- Drinking Bottled Oblivion
- Big Rat Grind (1.01 epa, used to be 1.77 epa, easily locked out of, but can be brought back w/ Fate)
- A thieves' cache! (1.48 epa |3 progress| or 1.41 epa |2 progress|, supplies via Connected: Docks and the Tankard| Since the revamp of Connected: Docks into Favours: The Docks, this works differently, though it is potentially more profitable. See sub-heading below.)
- Deal with mirror-smugglers and Kill the Fungus-column (~1.43epa, easily locked out of) (Used to be ~1.9epa but with Deal with mirror-smugglers becoming locked out at a level 8 of reduced the epa substantially.)
These aren't grinds, classically, but are here to show you how many echoes you're earning.
- A furious and incoherent drunken rat (4.14 epa [with a mood & max shadowy], spends SiC, incl. actions getting SiC ~.93 epa, not including rare success, including rare success 5.5~3.2 epa)
- Edit a book of meditations on the Anchoress (1.47 epa and a Scandal decrease)
- A Shrine of the Deep Blue Heaven (1.34 epa |3 progress| or 1.27 epa |2 progress|, supplies via Connected: Docks and the Tankard)
- Pygmalion:Lyme (1.22 epa in Glim, (30.8*111+700)e/33.8a)
- Beastly Breeding: Somnolent Hyaena(1.3~1.22 epa, depending on breeding method)
- Thefts of a Particular Character (1.26 epa [with a Gang of Hoodlums])
- Velocipede Squad (1.23 epa)
- Foreign Office (1.2 epa)
- Pygmalion:Devil (1.19 in Nevercold Brass, (34.1*111+1000)e/40.3a)
- Unfinished Business: A sure bet (1.2525 epa)
- Unfinished Business: Shake down your agent (1.25 epa)
- Unfinished Business: Eavesdropping, The prince of..., Doing the decent thing., Wade into the Ring Fights (1.21 epa)
- Unfinished Business: any other option that gives out rare successes (1.15 epa)
- Unfinished Business: the other options (1.1 epa)
- Unfinished Business: The study of antiquities, A lovely thought, An admirer among the clergy, A literary sort of fist-fight, Contracts and legs (1 epa)
- Unfinished Business: The anatomy of a secret, Mischief and brass (~including Church loss) (.95 epa)
Unknown EPA Edit
- Robbing various locales
- Duelling the Black Ribbon
- Acquiring Exhibits for the Labyrinth of Tigers
- Fate Velocipede Squad
- Fate Foreign Office
- Fate Apis Meet
Only listed are cards that provide higher than 1.5 epa.
- A Polite Invitation: Without drawing any opportunity cards during the party, taking Favour in High Places when possible, and cashing in Favours: Society, the direct yield is 13.05 echoes on an average of 7.44 actions, for 1.75 epa (better may be possible). Moreover, it provides up to 7 additional card draws (starting the party with 3 cards available to draw, and being a subscriber), which increases the expected EPA of future actions.
- The Bonfire of the Decencies (7.5e, but requires high bizarre, 6+ bizarre give greater then 5e)
- The Listing Tower: a Half-Abandoned Mansion (5e, but with a very difficult Dangerous challenge)
- The Neath's Mysteries (4e, only drawn at your lodgings [2.5e with a different option])
- A Visit (with relevant base stat at 200, 4e from Repentant Forger or Wry Functionary, 3e from Sardonic Music-Hall Singer, 5e from Regretful Soldier; the Soldier gives one Favour while the rest give two)
- All fear the Overgoat! (2.5e or 3.2epa [25e/8a] on success)
- Call in favours in the Flit (28.8e, [7.2epa, 28.8e/4a] or 25 scraps + Bundle of Oddities with reported max value of 230 [12.17e of scraps + 1.64e average value of oddities = 13.81e/4a = 3.46 epa], needs to take account of menaces, or verify the relevant Favours can be acquired without them)
- The High Castle: What Occurs in a Rooftop Shack (3e, if you go with the Repentant Forger)
- The Paronomastic Newshound (2.5e)
- The mechanics of progress (average modified Watchful pence [in Cryptic Clues])
- Promenade with the Celebrated Artist's Model (2.5e + some Making Waves)
- Ask the Sardonic Music-Hall Singer to help you (2.5e, going with His Amused Lordship. 1.8 epa [3.6e/2a] trading acquaintance with the Repentant Forger for with the Sardonic Music-Hall Singer)
- Wolfstack in the fog -> A huddled form (2.5e plus a Bundle of Oddities from Find him a job, only in Wolfstack docks with Airs 0-75)
- One's public (2.3e, but with a very difficult Persuasive challenge)
- Once upon a time in a carriage (2.1e)
- The Lofty Tower: the Potential of Premises at the Bazaar (2e + 4 scraps [3.7e incl. scraps])
- A day out in your Clay Sedan Chair (1.8e)
- What will you do with your Grubby Kitten? (1.7e)
- More Larks with the Young Stags (1.54e)
Rare Frequency Cards:
- A disgraceful spectacle (12.5e)
- The mournful rubbery instrument (3e, only in Spite without rare pet)
- A commotion above! (2.9/2.5/2.2e, only in your lodgings)
- A flash of white (2.5e, only in Ladybones Road without rare pet)
- A Raggedy Creature (2e, only in Veilgarden without rare pet)
Grinding scraps to trade in with one of the Relickers for one of the Tier 8 (1562.5e) items only works if you limit yourself to 3 scrap or higher actions.
With revamp of Favours: The Docks, the number of actions which have to be spent on expeditions, which is the only factor which the player can control, can be minimized. There is a section for Shrine of the Deep expeditions in the page which has been linked to in the sub-heading above. The final Echoes per action is, of course, still dependent on the reward which is obtained through Unpredictable Treasures.
There are four readily repeatable sources of Favours: The Docks, all via opportunity cards:
- Buy a round at the Rusty Tramp on By the River's Side: the Docks: Costs 10 Rostygold.
- Teach her to listen on What will you do with your Taciturn Mynah?: No direct cost, but precludes using a Partisan Messenger Tortoise to get Favours: Revolutionaries to cash in on Call in favours in the Flit.
- Visit him in his lodgings from Visit your acquaintance, the Regretful Soldier, from A Visit. No direct cost, but precludes (at that time) visiting one of the acquanitances that give 2 Favours.
- Stick to something stately when Time Remaining... = 3 after receiving A Polite Invitation. Only gives a Favour an estimated 25% of the time.
So the least costly is almost certainly Buy a round at the Rusty Tramp, so we'll go with that to start. Similarly, for A thieves' cache!, we'll assume that the source of the Favours: Criminals is Provide an alibi (criminals) or some other source that generates +1 suspicion.
For A thieves' cache!, the costs are:
- 1 Favour: Criminals, which requires 1 Action to generate and an average of 1/3 action and 0.5E to remove the resulting suspicion using Ablution Solution.
- 1 Action to start the Expedition and generate 1 Progress.
- 3 Actions and on average ~8.45 Supplies to generate the remaining 9 (or more) Progress, when taking into account the 5% probability per action of being able to play A sign?.
- Each Supplies consumed costs an average of 1/2 action and 2.5 Rostygold to replace.
- Two actions to finish the expedition and cash in.
And the expedition generates, on average:
- ~70% chance of 400 glim.
- ~30% chance of 500 souls.
- Unpredictable Treasures worth ~26.98 on average.
So the net profit is approximately 26.98 + .7*4 + .3*10 - 8.45 * .025 - 0.5 = ~32.07E.
And the total number of actions taken is approximately 4/3 + 1 + 3 + 8.45 / 2 + 2 = 11.56 actions, for an estimated 32.07/11.56 = 2.77 EPA.
Then there are also these Nadir cards
- The End of Battles (62.5e)
- An Unlikely Garden (62.5e on success, ~16% success with Persuasive 200 and max non-Mood gear bonuses)
- Old Bones (with A Daughter in the Shadows, ~12.5e [kinda])
- Woods in winter (with a Tattoo [from the Feast of the Exceptional Rose], 1cp of dramatic tension)
Without question the best echoes per action in the game is this . . . so long as you ignore opportunity costs. Otherwise, not so much. 10 Fate can be spent (among other things) to refresh 20 actions, which means that spending 50 Fate and 1 action is at least equivalent to spending 101 actions. So if you were going to spend that same amount of money on Fate anyway, and burned it all on action refreshes, you'd still make 165.64e (AotB) or 176.75e (Spirifage). Thus spending your Fate on A Special Delivery only gets you a further 146.86e or 135.75e, which doesn't sound quite as impressive. Even if it does take a lot less time and clicking. And since buying large amounts of Fate in order to spend it on grinding is a terrible idea (real-world) financially, well . . .
That's not to say that spending Fate on A Special Delivery is necessarily a bad idea if done the way it's intended to be, to speed up the process of gaining access to the Nadir. It's just a prohibitively expensive and inefficient way to grind echoes. There's much more efficient ways to convert Fate to money, such as doing Gallery of Serpents expeditions.
(As a point of reference, grinding the 11712.8e for an Overgoat this way would take 37.48 actions and 1874 Fate, or $334.64 [even with the most efficient Fate purchase at 280 Fate per $50]. Cider would take 512 actions and 25600 Fate, or $4571.43 - you could achieve immortality in under a week. Financially speaking, though, you'd be better off waiting for another Cider auction.)