Total Cannabis Sales Reached $56.4 Billion in 2016, $6.9 Billion or 12 percent occurred in legal transactions.
Cannabis is largely bought and sold outside of legal transactions in North America. 88 percent of the projected $56.4 billion total cannabis sales in 2016 were illegal according to ArcView's latest executive report. The cannabis investing firm projects that legal sales will grow to 21.6 billion in 2021.
The cost of keeping cannabis illegal is illogical from an socioeconomic vantage point. There is a great burden that incarcerating individuals for cannabis possession or distribution has on society. Society is missing out on the wealth that the cannabis industry offers.
Marijuana arrests account for over half of all drug arrests in the United States, according to the ACLU's analysis. 8.2 million cannabis related arrests were made in 2001 and 2010. 88 percent of these arrests were for possession. 
The tax burden of incarcerating one person for a year in 2010 was $31,307. The price is much higher in other states. In Connecticut, Washington state, and New York, the cost ranges from $50,000 to $60,000. [Price of Prisons PDF]
Incarcerating a citizen not only taints the reputation of the individual, preventing them from getting certain jobs. It prevents them from generating any economic activity for the period that they are incarcerated.
As if the effects incarcerating people for marijuana were not bad enough, the USA has been missing out on the wealth, jobs, and tax that the industry could generate.
“Imagine how much wealth will be staying in our country [the US], which is currently exiting, going to [drug] cartels in other countries, leaving our economy.” said Leslie Bocskor, an investment banker and president of advisory firm Electrum Partners. 
Fortunately, the days of illegal cannabis in North America are numbered. Here is some information from page 11 of the Executive Summary: