Fancy Guppies would be one sure bet as there always seems to be a demand for them (I'm a Dai Gurren dan/SRG Guppy fan. BTW, I've heard of pairs or trios selling for 100$!). However, you should try looking for something showy if you really want the cash. As a general rule, bigger, cleaner fins and brighter colors within a line/strain would likely fetch more.
Bettas would be another good bet, but you must be willing to have the space to separate the males. That aside, you could just pretty much sell 99% of the fry once you've raised them to adulthood, as you'd probably only need at most ten fish from the hundreds of fry produced. Oh, and I've seen Betta pairs here in the Philippines go for a 1000 pesos which would be like close to 23$. That plus quantity of produced fry would give you some good cash.
The only general advice I could give you on these is that you should expect to sell the fish you breed at least 10% lower compared to how much you bought it (If you bought from retail that is). Probably one of the better price indicators.
Say you bought a Guppy Trio for 20$. Advice would probably for you to sell the Gups at between 12-18$. It should be reasonable, and with the quantity, you'll more than make up.
But this is only personal speculation. I'm sure we have more experienced advice here, right?