I think you are confusing something. Sure, the vegan diet in rich countries is pretty expensive because it is specialized and produced in comparatively tiny quantities. Meanwhile, in the same countries the meat & dairy industry is heavily subsidised and can produce much cheaper. So cheap in fact, that there are incredibly high numbers of milk farmers committing suicide (e.g. see France).

However, look at poorer countries and you see that the cheapest is actually plant based food and meat is actually a luxury. This will additionally lead to more problems because these poorer countries are starting to consume more and more meat because they catch up to the richer countries.

In general, what is expensive and not is often determined by economy, production capacities etc. But if you compare plants vs meats, keeping animals is nearly always inefficient because they use roughly about 80-95% of the invested energy for respiration and their metabolism. Eating the plants directly would cut out the middle man (middle animal if you will). Btw the vast majority of soy, wheat and corn is for animal agriculture, and not for direct human consumption or for biofuels.

But you are definitely correct about vegan food as it is produced in rich countries is not the way to go. But you could say the same for pretty much anything we consume over here. Scaled up to global dimensions, it would be simply impossible for everyone in the world to eat the same amount of meat and dairy as in the US. There is just not enough available land on our planet.