If you want to breed sea creatures, Freshwater Stingrays is where the money is at. I've debated for a few years now to invest in some black FW Rays. Once Rays start breeding, they don't stop, and they have tons of pups.
One problem is the investment in them can be very costly. You can save in some ways. If you have an open large room, buy a 500 gallon swimming pool or something from Target or Wal-Mart. Cheaper than a glass aquarium and is is actually more suitable for Rays. Get yourself some nice filtration, as Rays eat a ton and mess up water parameters quick. The cost mainly comes from the Rays. The Black Rays can fetch $2000 a piece. The reason for this is because the exportation of Black Rays has been banned from all countries that have them. You want a Black Ray you pretty much have to goto a breeder or find someone rehoming them.
Another issue is they are a delicate species. I recommend starting with Motoro Rays. Although they grow much larger, they are very hardy for Rays, and fairly affordable. I highly recommend not starting with the Reticulated Ray, which is usually what the LFS sell under the name "Teacup Ray." Although cheap, a novice Ray keeper probably shouldn't start there.
If you want to stay Saltwater, there are many Saltwater Rays but they can get very large for even a swimming pool. The Atlantic Stingray in Florida stays small and is a saltwater ray, despite many people putting them in FW. They will breed in the right conditions, but they won't fetch you much money. Maybe $50-75 a pop, because they are pretty much pests in Florida and can be had very easily.
The market for saltwater rays is also not that great. With the amount of water changes needed, roughly 50% twice a week for a 200 gallon tank with 2 Rays, it gets pricey.
Got me thinking about Rays again... dang it. 72 gallon reef might be on hold.
If I were to get into breeding rays as opposed to just owning them, I would do this.
Buy the biggest pool I can fit into my basement. Buy a very nice heating element and put guards around it. Rays WILL burn themselves. Setup a very large Algae Scrubber filtration system. This, if done properly, will end the need for water changes twice a week. I'd add a couple of lights, but nothing too fancy. I would keep it for Rays only. Plecos and other fish like to graze on the rays. If they lose their coating they will be prone to infection and in most cases, will die.
I'd start with a breeding pair of Motoros.
I would feed them a lot. They are pigs. I would be buying bags of frozen shrimp every week and tossing them into the tank, keeping the shell on for their enrichment. Other Rays enrichment foods would also be good.
Not too bad overall. If you invest in black rays and are successful, the price they sell for will outweigh the food costs. If not, at least you have stingrays which are awesome.