here are the best numbers regarding how much money is WASTED ON ENFORCING MARIJUANA LAWS.
http://www.norml.org/index.cfm?Group_ID=3381
60,000 individuals are behind bars for marijuana offenses at a cost to taxpayers of $1.2 billion per year.
REFERENCE: Marijuana Arrests and Incarceration in the United States. 1999. The Federation of American Scientists' Drug Policy Analysis Bulletin.
Taxpayers annually spend between $7.5 billion and $10 billion arresting and prosecuting individuals for marijuana violations. Almost 90 percent of these arrests are for marijuana possession only.
The state of California saved nearly $1 billion dollars from 1976 to 1985 by decriminalizing the personal possession of one ounce of marijuana, according to a study of the state justice department budget.
REFERENCE: M. Aldrich and T. Mikuriya. 1988. Savings in California marijuana law enforcement costs attributable to the Moscone Act of 1976. Journal of Psychoactive Drugs 20: 75-81.