I agree that making the same volume of food is probably cheaper if you're processing vegan only ingredients - but if you look at the amount of nutrients / $, vegan substitutes are nowhere close to meat
These animals consume the same products the vegan alternatives are made with... for months or years, depending on the animal.
It might be true in places without massive animal farms, but that's usually connected to more human attention which would make it even more unlikely.
How many people are living off of grass?
It's usually a special feed, which is composed of corn, often bones and other substances.