If everyone stopped eating animals, the prices of a lot of plant agriculture might become uneconomical. My mistake for bringing in the organic/not distinction, since almost all non-organic fertilizers use some amount of animal products too. It's just that organic ones almost certainly use animal products.

(Very) broadly speaking, you can grow grains and legumes at scale without soil amendments from animals, though IDK about cost. You cannot grow fruits and vegetables at scale without soil amendments from animal products.

Like Diogenes living on handouts in Athens and decrying the materialist way of life, you can enjoy an economical vegan diet only if the world around you is not.