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.
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.
- Collections of Curiosities via the Tomb-Colonies connection card (1.8 epa, 65e/36a [1.81 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)
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: Shake down your agent (1.25 epa, most likely)
- Unfinished Business: Eavesdropping, Ambush a few couriers for old times' sake*, The prince of..., Doing the decent thing., A sure bet, Wade into the Ring Fights (1.2 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)
*this might be higher
Unknown EPA Edit
- Unfinished Business: Encourage the honest folk of Spite to give you 'gifts' (1.05 epa, maybe)
- 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.
- 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 (4e with a base stat at 200, except for Persuasive & the Sardonic Music Hall Singer. Also gains connections, notably 1 Favours: Criminals with the Repentant Forger)
- All fear the Overgoat! (2.5e or 3.2epa [25e/8a] on success)
- Call in favours in the Flit (28.8e, [3.2epa, 28.8e/9a] or 20 scraps [8.6e of scraps + Bundle of Oddities])
- 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)
- One's public (2.3e, but with a very difficult Persuasive challenge)
- Wolfstack in the fog -> Crying his wares (2.2e from buying a Stained Red Velvet Gown at the Bazaar and selling to the ragman, only in Wolfstack docks)
- 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)
- Once upon a time in a carriage (1.65e)
- 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.
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 spending 50 Fate and one action to get a 312.5 echo item isn't 312.5 EPA, it's 3.09 EPA at best, if you were going to spend their same amount of money on Fate anyway, and burn it all in action refreshes. And since buying large amounts of Fate in order to spend it on action refreshes 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 terrible way to grind echoes.