Going to go off on a bit of a tangent, but it loops back to the topic.
You could do a similar analysis to determine the amount of food required to feed a chicken to create one store-bought plucked and pre-cleaned chicken, for example, and it would be similarly huge (I have other ambitions than doing that math at the moment), due to the fact that most of the calories an animal takes in are used to sustain their daily activity, rather than stored, and you cannot get anywhere near as much energy out of eating a herbivore as you can out of eating the plants the herbivore ate, if they are edible for you - and the efficiency is even worse if you're eating a carnivore that eats herbivores. (This is basically high school level biology)
The fact is that we're not living in a feudal society where you grow your own plants, have your own animals, and have them graze on your pasture or whatever, and anything you need you grow yourself, and then your feudal lords take most of your food and leave you with enough that you can live in perpetual near-starvation. Instead, we have giant factory farms that grow ludicrous amounts of plants, whether for corn or tobacco or other crops, some of which go to feeding animals that become additional foods (some graze on pastures, but there may not be enough, especially in a drought), so that anyone in a first-world country who can afford it or has government assistance can acquire more food than they could have possibly grown and kept themselves in the old days (in addition to not having to take the time to grow any of it).
Why is marijuana so expensive (compared to, say, cigarettes, even with taxes)*? For one, it can't be readily factory farmed anywhere it is illegal, so there's little to no economy of scale for it except possibly in lawless places or places where it is legal (I suppose, but that just doesn't show up at all in the news here - as though they don't exist) - and there are the costs of avoiding the law, whether it is grown here in secrecy, or shipped in surreptitiously. [I make lots of assumptions in this paragraph! It is chock full of assumptions! Whee.]
* I assume the price has not gone down, at any rate.