Do we know what the OPs water stats are like? He is hardly going to be able to breed neons if he has hard alkaline water. A few suggestions might be:
For hard water:
small slightly less common livebearers, such as limias, heterandria formosa, merry widows
shell dwelling cichlids
shrimps
possibly pseudomugils or galaxy rasboras
for soft water:
cories
tetras
I very much doubt you would be able to make much of a profit though: I breed some of the above and I find I have to give them away these days if I can't sell them at auction; local shops just aren't interested in sporadic contributions from hobbyists
to make it pay you would need a large outfit that churned out regular, predictable supplies of some fish that the shops were anxious to sell
I don't see that happening with a single small tank.
Having said that, I wouldn't discourage anyone from breeding: it is a great way to learn about fish. Just don't assume you are going to recoup your costs.