http://www.cato-at-liberty.org/2009/03/12/end-drug-war-and-s...
http://www.theregister.co.uk/2009/04/07/legalising_drugs_rep...
In any case, enforcement savings would long before taxes. Possibly alone. Outright legalisation is rarely really on the table. It's decriminalisation of personal use, reducing it to rarely enforced minor infringement or something along those lines.
You can't expect the full suite of benefits from that. You save quite a lot on enforcement and incarceration. Mostly, the proponents want that to go into treatment.
But you still have associated crime, Criminals still control the industry. You don't get any taxes.
As pot smoking is a fairly social activity, I think many people would happily buy it over a counter - although ironically, California's strict anti-smoking laws might be a bigger barrier to the spread of such establishments. Personally, I much preferred the Dutch model - same way I can buy beer at the supermarket, and it's better value, but oftentimes it's nice to go to a bar and get it on tap. Bars don't seem to be limited by the existence of liquor stores.
In the case of the most addictive substances that tend to drive a lot of petty crime it's probably better to give the stuff away free.
Decriminalising personal use (but not sale, import, cultivation, etc.) doesn't fundamentally change that.