There are nine US states with legal recreational marijuana; five on the west coast and four in the east.
Pricenomics, acting on behalf of Wikileaf, priced out the cost of an eighth of an ounce of weed in major markets on both coasts, and found that weed is 22.5% cheaper out west, though there's a lot of regional variation (San Franciscans pay 50% more than Portlanders). Some of the price difference can be blamed on oversupply (weed can't be legally transported across state lines, so oversupplies don't smooth out), but the rest is down to local taxation and cost of doing business.
That the supply of marijuana cannot legally cross state lines explains only part of the variation in prices among cities. Another reason why prices are different by city (even in the same state) is local taxation laws and the cost of living in an area. Each state typically levies an additional sales tax on marijuana, in addition to local city cannabis sales taxes and regular sales taxes. In some cities, that can add approximately 35% more to the prices. That combined with the higher rent and salaries associated with expensive cities like San Francisco, mean the cost of cannabis in some Western cities can rival their counterparts in the East.
Here's How Much Marijuana Costs on the West Coast vs East Coast [Pricenomics Data Studios]
(via Naked Capitalism)