Barring extreme over-regulation that limits supply, we can expect to see legal pot prices fall rapidly. The illegal market provides enormous profits to all levels of distribution as a risk premium to compensate the growers and dealers for the risk of getting imprisoned. Obviously, the profit margins on legal pot grown in a competitive market should be much lower. Further, the costs involved in keeping an illegal grow-op inconspicuous are very large. The cost of production of marijuana legally grown in a hothouse should be no more than a few dollars per kilogram, and the cost of production of field crops should be even less than that. 

The current medical marijuana producers are making windfall profits despite the huge costs of compliance with absurd "security" regulations (there is no way that farming should be done in warehouses). Get the security costs out of the way, eliminate the constraints on production, and watch the price plummet.