More than we know.

Even if everyone grew their own marijuana and no large tax boon was realized,what about all the untaxed cash flow now going to Mexico,being spent in our own country because we no longer have to buy marijuana grown by the Cartels.
And the amount of money saved by people that replaced some of the pharmaceutical prescription drugs ,such as sleeping pills,would be staggering.
But that is exactly why big pharm is fighting legalization or even removing marijuana from schedule1.
They spent 30 million in the 2008 election ,and I'll bet they top that by double in the 2010 election funds.
They don't follow any political agenda and in the last cycle they gave too both parties equally.
http://www.opensecrets.org/industries/indus.php?ind=H04
The above link will let you check to see if your legislators are receiving funds from the pharmaceutical industry and will tell you how they will vote on any decriminalization of marijuana.
Ron Paul and Barney Frank ,with bills in congress for marijuana legalization,receive no funds from the lobby,while both my senators from Texas receive hundreds of thousands from them and both refuse too support even medical marijuana legalization.