Some species of Corydoras catfish are insanely easy to breed. Get males and females in a 2:1 ratio. Feed them live blackworms, frozen bloodworms, high quality sinking pellets, and the occasional spirulina tablet. 10 gallons of tank per inch of your largest female. When the females are getting rounded out, stop water changes and wait for a weatherfront to come through. Then do a 50% water change with cool, soft water ideally just as the front passes over, but you have some leeway. The water change should drop the temperature of the tank 5 to 10 degrees. That's for the medium hard varieties of cory. When you see spawning behavior (and it's obvious once you know what to look for) stand by and, when the female has had enough, remove the eggs to a hatching tank or net with some anti-fungal (methylene blue or dried alder cones work well)
Regular and albino Corydoras aeneus, C. paleatus, C. sterbae, C. panda, and C. pygmaeus have all bred in my tanks with no intervention from me other than using slightly cooler water on regular water changes. I have friends who have bred C. sp. CW 010 and 009 (Gold and Green Laser), C. oiquapensis, and Brochis splendens with no more effort than that. I've also got friends who have bred C. haraltschultzi, C. ernhardti, Scleromystax barbatus, and Aspidoras pauciradiatus with not much more difficulty.
So Corys are an easy one.
Show grade guppies, if you're willing to put $50 into a pair (and you really ought to buy at least 2 pairs) will pay for themselves IF you keep the males and females separate except to breed, and cull any imperfect specimens.
Angelfish, done properly, are easy to breed and will pay for themselves (but you really need multiple tanks to do it properly).
Mollies are no where near as easy to keep as people make them out to be (they have zero tolerance for ammonia or nitrates and require exquisitly clean water, which with vegetarian poop machines like mollies can be difficult to maintain.) They're also not as easy to sell as you might expect and don't bring in much money.
Some of the rift lake cichlids can pay for themselves - good quality Aulonacaras, for instance.
White clouds in a species tank will breed in a colony set up. Long finned white clouds actually make a pretty decent return.
Some killifish are a pretty good deal too. Aphyosemion australe and A. striatum and Fundulopanchax gardneri are extremely easy to breed (1 male, 2 females in a tank 1/3 full of java moss, with a sponge filter and a good cover. Trade out the moss to a hatching tank every week or so. You'll get plenty of fry.)
Most rainbowfish species are easy too. But you need a good sized tank for the breeding stock, and some good, small fry food (infusoria or rotifers or microworms) for the free swimming fry for the first couple of days.
There's a secret to breeding fish: you don't. They breed themselves. All you can do is give them what they need to be happy. Sometimes that's really easy. Sometimes it's really simple, but very hard in practice (a friend of mine breeds stingrays. All they need is a sand bottom, lots of high grade meaty food, exceptionally clean and soft water, and about 500 gallons of space per pair at least 3' wide in every direction.) Sometimes breeding is easy if you can get the adults away from the eggs before they eat them (most cyprinids are like that. You'll get a good hatch if you spawn the adults in a tank over a grid that will pass the eggs but not the adults.) Sometimes the problem is that you need to leave the parent in, but not too long. Bubble nesting bettas and gouramis are like that - the male will take care of the eggs until shortly after free swimming, but after that, they'll start eating fry. And sometimes you just get huge dies off of fry -again with the labyrinth fish, a lot of fry the labyrinth organ doesn't mature properly and they die.