Now, there are alternatives. If we were to decriminalize marijuana (users would be simply fined, like a speeding ticket), you would probably realize 90% of your projected "savings". Why do we have to go to the extreme and legalize in order to save money?
Money is not the only resource. The time, effort and resources of law enforcement, courts and others to deal with marijuana suppression, even as a "decriminalized" issue are still going to be invested into marijuana control. When you decriminalize a substance that does not mean trafficking in it will become legal. It will still be illegal to grow it and sell it. That diverts resources. Lets just legalize it, regulate it, tax it and forget it.