Really depends on how you do it. I think a lot of the revenue numbers are highly exaggerated. Mostly because they are based off of the illegal price. Pot is not much harder to grow than tobacco or other crops and requires much less processing to be ready for sale.
In a fully legal environment I would think prices in the couple dollars an ounce range would be about what you would see. Now way someone will pay hundreds for something that you plant and wait for it to grow.
Sure you have your people who want high end hydroponic hybrid shit, but they are the minority. And really its not going to be harder then say designer orchids which are often hybrids or clones which require very precise growing conditions. Even then a hundred bucks for a whole plant is considered very very expensive.
Now you could just stick a 20-40 dollar tax per gram on it, but then you have to enforce that and you lose some of the saving on that end. Plus you are right back where you started throwing people in jail. I mean I'll go to the store and buy corn, because growing it is hard and its cheap. But if for a little effort someone can grow something that will save them hundreds or thousands per year, you'll need some stiff penalties to discourage that or even sales only in state stores. Somewhat like we have with liquor except your dealing with something even easier to produce at home.
I love the idea of legalization for rights and savings issues, but as a revenue generator, I think we'll be a bit disappointed.
Just ask yourself what it really costs to grow marijuana in terms of land, labor, and seeds, plus a small profit margin.