Legal cannabis will naturally be much, much cheaper than illegal cannabis. A joint is the same sort of item as a teabag: the dried flowers of a plant in a wrapper. A fancy teabag costs a dime at the supermarket; the marijuana in an average joint costs about $4 (0.4 gram of sinsemilla flowers @ $10/gram) on the current illicit and quasi-medical markets. The combination of not having to worry about law enforcement and the economies of mass production will inevitably drive the joint price down close to the teabag price.