A lot. Marijuana prohibition costs enough to put more than 900 new police officers on the streets.
Here’s the numbers from Harvard and Cato scholar Jeffrey A. Miron and marijuana law reform researcher Abhi Sivasailam (via Show-Me Cannabis):
Missouri would save about $90 million in government expenditure and yield roughly $59 million in tax revenue annually. This assumes that marijuana would be taxed at rates comparable to those on alcohol and tobacco and that all other states and the federal government would also legalize.