Profitable things to breed are not expensive fish. Expensive fish are often either wild caught, or cost a lot to raise from egg to sellable size. Great money makers are corydoras species, sailfin mollies, discus, african cichlids. Corydoras grow very quickly, and can be turned around with minimal effort. Sailfins are often in short supply and grow moderately fast. Discus fetch a high price, but unless you have many different strains that are unique, then you probably won't sell too many. African cichlids are good sellers, and are somewhat easy to breed. If you really want to make money, it depend on your demographic. In my area I average $100-150 per month in plants clippings and ramshorn snails. This doesn't include my sailfin mollies, or cories that I breed. I also have Malaysian trumpet snails I sell on occasion. Current projects include corydoras adolfoi, which are great money makers (but weitzmanni are better money makers), c. sterbai, green phantoms, yellow labidochromis, elongatus, and discus.
In the end the most important thing to breeding is that you have fun. If you want to make money, you need to look at more than how much they sell for, but also, what it takes to get a good batch of fry to adulthood/sub adulthood, what cost are involved with raising the young, what special foods are need to condition adults, any special additives (spawning caves, sand blackwater), how many you will be able to sell for money in the local area, and how much you can sell them for.