Much bigger than the direct costs of arresting/prosecuting/imprisoning people is the economic loss they represent -- assuming those people could do something productive, they'd be contributing a lot more to society than they cost to lock up today. On top of the costs of locking them up. And, the competent people currently working in law enforcement who could be doing other things. And the deadweight losses, etc. from people (rightly, in many communities) being distrustful of law enforcement and the government.

Ending the drug war would be a positive of around the same order of magnitude that the War on Terror has been a negative; hundreds of billions per year on an open-ended basis.