The problem is that they wont want to make it legal for a “meager cost” and one plant yields too damn much of it.
Most users wouldn't need more than a plant or two a year.
They wont make it legal to get $10 or so year in taxes per user. Much over that and people grow their own.
Nonsense. Look at the medical marijuana dispensaries in California and the “coffeeshops” in the Netherlands. These places do gangbusters business selling expensive pot to people who could grow their own. Medical marijuana card holders can grow their own. The Dutch can grow up to five plants and not worry about being arrested. Yet the coffeeshops and the medical marijuana dispensaries do big business and in both cases the government is collecting sales taxes. California is already bringing in millions of dollars a year on sales taxes on medical marijuana. These shops will sell pot at prices up to $20 or $30 a gram or more to people who could just grow their own. Hardly any grow their own though. They walk into a nice clean shop and select from a wide variety of quality product because they don't want to mess with growing their own and they like the variety these shops offer.
Back during alcohol Prohibition people would drink just about anything. The quality and the taste really weren't big concerns because people didn't have much choice. They had to take what they could get. Then when alcohol became legal again people found their favorite brands and not many wanted to mess with the stuff some yahoo was cooking up out in the woods. They wanted store bought alcohol that was produced in a regulated industry. These coffeeshops in the Netherlands and the medical marijuana dispensaries in California buy their product from the black market. When the shops are purchasing their product from a regulated industry even more people will want to go to them because they will have some assurance that the product they are buying is not moldy and it doesn't have toxic pesticides or other chemicals on it. In time people will have their favorite brands or varieties and they won't want anything to do with crappy homegrown.
You said pot is as easy to grow as tomatoes. I doubt it's that easy because it takes longer to produce and they have to pick the pot at just the right time and dry and cure it just right, but there is no reason why it should cost hundreds of dollars a pound to produce it other than the fact that it is illegal. In a regulated environment it will be grown on large farms by farmers using modern agricultural methods like they use for other crops. They'll mechanize the process as much as possible. They'll find uses for the whole plant and not just the buds. They'll bring production costs down to a fraction of what they are today. More importantly all the added costs associated with the risk of arrest and seizure will be gone. There won't be so many middlemen who all want to make a hefty profit to justify the risk they are taking. There won't be people getting paid a few grand to drive cars packed with a couple of hundred pounds of weed. A truck driver will be hauling many tons of it in something like a Budweiser truck for less money than the mule who hauls a couple of hundred pounds gets paid.
In no time wholesale prices will drop through the floor, and they'll keep dropping as competition forces producers to cut costs further. The only way we'll be able to keep retail prices anywhere close to where they are today is with high taxes. Even with really high taxes the costs to consumers could still be considerably lower than they are today. They're going to be able to walk into a nice clean store and choose from a wide variety of reasonably priced quality product even when taxes are figured in. We will clean up on tax revenues, on sales taxes and excises and all the taxes on the incomes of law abiding Americans working in the regulated industry and the taxes they pay on the things they buy with the money coming from the regulated industry.