That must be some special plywood, for the $150 difference.
I would hope it's marine ply, it's not cheap, but it's not worth *THAT* much used. $100~180 for a 1/2~3/4 inch sheet of quality marine ply, probably take 1.5 sheets. Normal ply (and some cheaper marine ply's) will just de-laminate outside in a setup like that roofrack.
But who knows, it could well be the normal $30 stuff.