I assume the tank is 48 x 24 x 24? The number of breeds of stingrays you can put depends on the type of stingrays and how big they get. Remember that they are bottom dwellers, so its really the bottom dimensions of the tank that you're interested in. The depth is pretty much useless for stingrays. Near me, I ofter see motoro stingrays for sale, which get to be about 18" across, so a 24" tank bottom doesn't give them much room to swim.
There are quite a few different breeds of freshwater stingrays, and though they may be sold as juveniles in the local pet store, they can grow to be 3 foot wide, if given proper care. Stingrays are very delicate fish and require a lot of care, attention, and expertise. They are EXTREMELY sensitive to water parameters. How long have you been keeping fish? what kind do you keep? Only an expert aquariest should try to keep them in captivity. They can also be very dangerous, since they DO sting and have poisonous stingers.
I would strongly suggest getting a good BOOK (check amazon.com) on keeping stingrays and reading it carefully to make sure you are ready to commit that much time, effort, and money to them. There doesn't seem to be much info about them on the web, but you acn try looking at :
http://www.geocities.com/Heartland/Meadows/5590/care.html
http://www.seremban.net/fishcorner/fc310799.htm
As for food cost, it depends what you feed them. They prefer fresh food, such as small fish, and seem to love store bought shrimp. Variety seems key to health. Some rays cannot catch their own food, and need to be fed dead food. I read somewhere about a breeder that says he feeds his rays 2X a day, but as for an exact dollar amount, it would greatly depend on the species, size of the ray, and what you are feeding it.
As for equipment, make sure your heater is out of the ray's reach or has a shield on it, rays get easily burned. What are the capacities of the canister filter and the powerheads?
Substrate: rays need substrate that cannot injure them, so sand is most highly recommended, but is also harded to clean than gravel. Many people keep rays in barebottom tanks, the only setback to this is that it isn't so pretty. People also keep rays with very fine, (so they can burrow) round (so they don't get injured) gravel.
You should know A LOT more befor eyou try to keep rays. It would be a big waste of money and a disappointment to go out and buy these expensive and beautiful creatures just to bring it home and have it die quickly. I am only giving you info I read online or happen to know from my
lfs. I am in no means an expert, but I strongly suggest you find a LOT more info before trying to keep rays.