What do want on the top? Blocks, bricks, plain concrete or whatever? Have you considered a gravel finish? That whitish gravel looks quite good. I've always fancied those white seashells that scrunch when you park the Ferrari
Is it fairly flat or on a slope? Depending on your ground conditions I wouldn't think you have to go down 10 inches. May be a concrete screed 4 - 6 inches deep and then whatever you decide on top. dl