Harvard study shows 500 economists agree pot legalistion a great idea
Here is an incredibly detailed analysis of the question of exactly how much would be saved in the USA if they went to full legalisation.
The report examines the budgetary implications of legalizing marijuana – taxing and regulating it like other goods – in all fifty states and at the federal level, and estimates that legalizing marijuana would save $7.7 billion per year in government expenditure on enforcement of prohibition. $5.3 billion of this savings would accrue to state and local governments, while $2.4 billion would accrue to the federal government.
Miron Report (PDF) 2005