Don't go with relics even tho they stay small and are very common. Most reticulated rays are very unhealthy. If you can find one that's been in captivity for a while and is eating I'd say go for it seeing you can keep a pair of retics in a 180. But most of the time when you see retics in a pet store there in bad shape. A good thing to look for when buying rays is look at the base of there disk where there tale is if it's fat it's healthy if you can see its hips (looks like two bones sticking up) walk away. Rays are awsome if you can house them correctly but as alot of people on here say you do need a big tank. Motor's can get up to a 30" disk
You're telling me rays that are imported from South America are less than $100? I'm sure they must be well treated to fetch such a price.