because it depends on human action. It would probably be less expensive because more people would grow it, but I doubt it would become as cheap as parsely because it has much greater utility. Manufacturers would start using it in new applications, and that would put upward pressure on the hemp price and take pressure off of the inferior substitutes that they currently use. All we know is that everything would be different.
Please don't interpret this as an argument against legalization - the war on drugs is unbelievably stupid- but realize that under full legalization, a lot of other things that grow from seeds would become more expensive, because many farmers that had been growing something else would be cultivating acres of MJ instead. I think lots of people would grow it for their own personal use, but in order to sell it, they would have a hard time competing with the mass-produced stuff in an Up-and-Up bottle.