Are you sure most pot used here comes from the US? Our government isn't so sure about that. Most estimates are that more than half comes from Mexico. They believe that in 2007 15,500 metric tons were produced in Mexico and most of it came here, and more was produced there in 2008, again with most of it coming here. There are parts of this country were mostly what you will find is domestic grown, but Mexican can be found most everywhere and throughout much of this country it completely dominates the market. They believe the total amount available on the market here in a year, foreign and domestic, is between 12,000 to 25,000 metric tons, and that Mexican pot could make up well over half of what is consumed here.
First are you sure you want to use Government estimates on ANYTHING, especially with something they are involved in? they have a vested interest in staying on the "war on drugs" path so of course they want you to believe most of the stuff comes across the border.
But it has been known for many years that most pot is grown and consumed within our borders. even though we have been at this 'war on drugs" for many years they haven't even nicked the cultivation within our own borders much less outside the country!
That is why the "WOD" is such as waste of money and manpower at the expense of your freedom... 30+ years and they haven't made a dent!
Second do a basic web search and you'll get your answer... but here is one of many articles...