"The UNODC (2005) estimates that the world retail market for cannabis was about €125 Billion4 circa 2003; more than the retail markets for cocaine and opiates combined. The US is believed to be the largest contributor to this estimate, but the exact size of that market is far from settled. Indeed, some of the estimates of the US market vary by a factor of 10.
"The UNODC’s macro estimates indicate that North America and Western/Central Europe account for 45% and 28% of the world cannabis market, respectively. The UNODC’s input-output model suggests that each past year user in North America consumed 165 grams of cannabis herb at almost €10 per gram. With approximately 25 million past-year users in the US during this time, the UNODC calculations imply that retail cannabis expenditures in the US exceeded €40 billion. This is more than four times the retail estimate generated by the White House’s Office of National Drug Control Policy for 2000." [all monetary values are in €2005]
Beau Kilmer, Rosalie Liccardo Pacula, "Estimating the size of the global drug market: A demand-side approach," Prepared for the Euopean Commission (The RAND Corporation, 2009) p. 8.