Wow, you didn't think this over did you?
First off, if we ever got to the point of legalizing marijuana we would use the Amsterdam model which includes extremely strict minimum sentences for selling it without a license, even worse than we have now for drug dealers that get caught. Everyone would be allowed to have a certain number of plants in their own residence (I believe they are allowed 3-5 or something close in Amsterdam).
Why don't you think there is an underground market for tobacco and alcohol? Because money would not be saved and the quality would be extremely under-par. The government would be able to tax the living hell out of it and make a ton of money off legalizing marijuana. The government makes somewhere around 30% of the overall price of the cigarettes in taxes, and I'm sure marijuana would be very similar.
Bottom line, there are many states talking about legalizing and taxing marijuana (California being the "example" state) because it will create more money for the state and in turn the federal government. It will also create jobs because the pot will have to be grown somehow. Then subtract the money saved from jail expenses on people with low level drug offenses, and money spent on enforcement (helicopters are a widely used anti-marijuana weapon), I can't see marijuana not being legal and taxed within about 5 years. The states need the money and this is a very easy and quick method to creating a significant amount of revenue.