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There are many different types of foundations suitable for spas but the one that is highly recommended is a reinforced concrete slab that has a minimum thickness of 4''.
A gravel bed? It sounds like a good idea because like you said, you may wish to move it down the road. Is that recomended?
TT,$15 per sq ft is 2 to 3 x higher than the going rate here in the STL. Have you collected multiple quotes? I think you'll be able to find someone out there to do it for cheaper. What size spa did you buy? If it's a smaller tub, you might be ok w/o rebar or some other type or reinforcement. The reinforcement materials really shouldn't elevate the cost too much.
Pack the gravel firm (rent a tool that will hard pack it down. For the life of me, right now, I can't remember what it's called.)