Probably the easiest thing to make your money back on would be shrimp. Cherry shrimp are fairly cheap these days, but you can still make good money on snowball, tiger, crystal red, etc. I would say to start a breeding colony all you really need is a simply set up 10 gallon, and perhaps 10 Shrimp to start with. They should do the rest. Of course, you'll need to sell them on aquabid or elsewhere online, but shrimp apparently mail pretty easy. If you figure a shrimp tank costs $40 to set up, and you buy 10 snowball shrimp for $5 each, you just need to wait till you have 18 offspring until the initial costs recoup themselves. After that, the project should be self-sustaining at the very least, as only a few shrimp sales per month ought to take care of food, electricity, and water costs.
Barring that, probably killifish would be the most lucrative option - mainly because in many cases you can ship the eggs dry and save a lot on postage. In this case you must rely on online sales however.
If you wanted to breed solely for LFS, I'd suggest Angelfish, as they aren't too difficult to breed, are fairly prolific, and common "newbie" fish.