Can I say "priset av" or is it always "priset på"?
It's priset på, but the combination till priset av means 'at the expense of'. Han lyckades till priset av sin värdighet 'He succeeded at the expense of his dignity'.
Why not "on the car"? What would you say if asking about a price listed directly on an item for sale, perhaps in the context of negotiating a lower price or if there is another special offer that would affect the tag price?
What is wrong with "What is the car price?"?