Prior to any state legalizing marijuana, economist Jeff Miron estimated that it would cause pot prices to fall by 50%. Legalized pot blew through that estimate in less than 24 months and is still dropping fast.
Illegal drug prices have generally fallen over time as industries become more efficient, and this fact has often been used to assert that prohibition doesn’t raise drug prices. That argument has been destroyed by the real time experiment of legalizing marijuana, as I explain in Washington Post Wonkblog.