Here is a good attempt at a cost benefit courtesy of Jeffrey Miron, Professor of Economics at Harvard. It also carries the endorsement of 500+ other Economists:
Costs of Marijuana Prohibition: Economic Analysis
I still cannot find the data on the MPP study (pretty sure it was theirs) which is used for the AB 390 estimates of 1.3 billion in rev. for CA, and also estimates Ca's expenditures. I really should have bookmarked that.