Most people would just grow their own unless what they sell at the store is the kush of kush. And for cheap too. If not the black market would supply the demand. The black market in Colorodo and Washington is probably booming right now.
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1 out of 8 are in jail for MJ, most all are in state and local jails. Leave it for the locals and states to continue putting potheads in jail along the lines of prohibition in NC...if a county wants to be dry they pay to enforce it.
But there won't be some great weight lifted off the federal budget. If legalized, the 5000 DEA agents and resources would be split into the ATF (or just now just DF, Drugs and Firearms) and IRS for revenue collection, Immigration and Customs to continue the fight against smuggling, FDA regarding illegal prescription drug use, etc.