I only pay around $100 to $110 for my Motoro Rays. They are usually somewere around 4 1/2 " round or so. But prices are different around the area..
Tank mates, well some that come to my mind are silver dollars, clown knife, good size clown loaches, Indo Dats, P-bass, discuss (just be careful on those so they dont become a $$ snack)
its just what you like, but the main thing that its something that will not pick on the rays disk. I myself try and find fish that eat about the same things as my rays.
So I keep an Aro, Indo Dats and Clown Knife in with mine. I do also have silver Dollars as a schooling fish for other things to be swimming around in the tank.
Ive seen some people do cichlids, but I myself stay away from fish that like to nip.
And of course, NO sucker fish.