It is dependent on availability, how desirable the fish is and personal preference, black rays are hard to find now and command a much higher price
Most of my rays have been bought from fellow hobbyists and are reasonably priced, have paid between £30 ( reticulatus ) and £300+ ( Itaituba )
Motoro's and Retic are probably the most common, hardiest and the ones you see in LFS, expect to pay between £50 and £100 for a retic or a motoro