These numbers will always be based on assumptions due to various factors. But unless you have something better to add, then I'm going to stick with the best assumptions we have so far. You do the math, I already did mine on page 1. And for your assumption that everybody will grow their own, well, look at Holland ok, and what a Dutchman has already told you in this thread. But it didn't really sink in, did it?
Since your assumption is based on a mistaken notion that excellent quality marijuana can't be grown in backyards, basements, sun porches, and garages for a reasonable price and in quantities necessary to support the demand, your assumption is flawed.
And I'm not sure why you cling to the mistaken belief that everywhere is like Holland. I happen to live within 150 miles of nearly 30 million acres of some of the worlds most productive agricultural land. That's three times more
agricultural land than the total land area of Holland. And the state to my west is just as productive. And the state to my east does almost as well. I could pitch a handful of seeds along the fence row of a pal's farm, come back in a few months, and fill the back of a pickup truck with a year's supply for everyone I ever knew who smoked pot. And if my pal wasn't risking having his land confiscated, that sort of thing would be common.
Marijuana costs about the same to produce as parsley, sage, rosemary, or thyme. Those can typically be bought by the ounce for just few dollars, and in bulk for less than 20 dollars a pound. And just like tomatoes, bell peppers, and zucchini, when the backyard gardens are harvested, lots of people will be giving it away to friends and neighbors.
If the tax scheme applied to legal marijuana brings the total price to more than people are willing to pay, they'll grow it. Of course that price point will be a subjective decision. Some won't want to bother with growing it. It will be less practical for some people. And only a minuscule fraction of pot smokers are the kind of hard core stoners you see going in and out of the dope shops next door to you, those few people who really are stupid enough to believe you can't grow excellent marijuana in a backyard garden.
Illinois has a 5-plant limit of having being busted be a Misdemeanor. Now, they call this "decriminalization," but a Misdemeanor is a criminal offense non-the-less. If they really meant this to be decriminalized, they would make it an offense against the tax act and fine you the tax plus a penalty and be done with it.
If there were no restrictions on growing just five plants per person, if people weren't risking losing their property for growing it, the marijuana black market would be effectively wiped out within a couple of years. Even without any taxation or fines, there would be a huge decrease in the state's financial burden that comes with enforcement of prohibition. That savings alone would be an economic step in the right direction.