Do you think the price of cars would go down or up if we made less of them and banned exports?
Why would we make less of them? We would ban imports too. The laws of supply and demand work for a domestic market too. Let's say a plague wiped out the rest of the world's population, total depopulation except for the USA. So we would just go without? No I don't think so. The domestic supply would come on line to supply ALL of our needs.