I don't where the demarcation is, but , e.g., I wouldn't care about paying a tax of $5 per oz. You slap a $100 tax per oz and I'd be growing my own even if it were dirt weed.
My point is that at some point a tax gets too high and provides a strong motivation to evade it. In this case we'd be back to illegal dealers selling black market weed or people just opting to grow their own.
I absolutely think there's a good deal of revenue to be had by legalizing and taxing if done correctly.
Fern
The taxes on cigarettes are really high in Canada (Nearly 40% effective rate looking at this rate table and an $11 pack of smokes after tax) and people rarely get black market smokes. A lot of it is because black market smokes are not nearly as good (or at least aren't perceived as good) as name brand smokes.
Now, weed is certainly easier to grow, but most still won't do it out of laziness. In addition, once you factor in mass production of marijuana (real mass production that comes with legalization not the piddly shit they do now) cost per ounce to produce is going to plummet. I wouldn't be surprised if the price of weed in the absence of any taxes would be less than $50 (or much less, I don't see why a pack of weed smokes would be that much more expensive than a pack of cigarette smokes) and you can tax it all to hell and it'd still be a significant discount over what it is now.