I don't think I'm doing too badly at the moment, only level 33 and got 265k, kitted my cruiser up to pretty good gear.. Thought about buying a fleet module to sell for 5+mill EC.. But doesn't really seem worth it, probably better to save FC for when I reach level 50.
Now that the DOFF missions for converting lower-quality DOFFs to higher-quality ones have had a hefty Dilithium cost added to them, it is more profitable to sell off your unwanted ones on the Exchange. Highly-demanded DOFFs that get consumed in Starbase/Embassy projects sell for tens of thousands of EC each. If you run all of the recruiting missions (from the recruiting officer AND from the race-specific Vulcan, Andorian, and Tellarite ones), then you can get over a million EC worth of DOFFs per week on a character.
How many Starfleet Engineers does it take to exchange an Anti-positronic Photon Emitter?