Why not contact geotechnical firms in your area and ask them about testing for relative density? They will have a schedule of rates for laboratory testing and I can't see why they wouldn't give to you - it is not like it is "secret" so if you are trying to know how much YOU should charge, . . .
Looks like BigH gave you some good insight. We don't do these tests in our laboratory. That said, you can call some of the other labs in the country and they'll tell you their cost.