Breeding for profit is extremely difficult because there's always a company or someone else out there that is breeding these fish in mass quantities and can sell them for lower prices. If you just want to breed enough fish to help cover some of the expenses of the hobby, guppies are probably the best because due to how the fry practically raise themselves, even then, the profit isn't much when considering electric cost, food cost, water treatment cost/etc.
Selling live foods seems to be a bit more profitable for the amount of work you will need to do because raising these cultures cost close to nothing and require little work and maintenance and can sell for anywhere between 5-10$ depending on the size of these cultures. (white worms, grindal worms, daphnia, copepods(scuds), microworms, vinegar eels, etc.) Also, you will now have a wide variety of live foods to feed your own fish.