I would guess (and that's all it is) that 35mm costs more to make, the perforations, the little colour-printed box, the plastic canister, the metal can etc. I doubt that's the full answer though. Maybe also, professionals shoot 120, and they *need* to make it work economically, hobbyists shooting 35mm *want* to make it work economically. Just a guess though.
Provia 400X really is quite remarkably expensive though, maybe it really is just showing that it's a low volume product.