The main goal for MREs isn't to be cheap, it's to be nutritious, shelf stable, and easy to prepare. There are certainly cheaper ways if your only goal is to be nutritious.
I made some burritos a couple weeks ago. Mainly rice and beans, with some beef, cheese, and salsa for flavor, seasoned to my liking in a flour wrap. The intent was to freeze them for quick meals, so no fresh veggies. One or two of those paired with a salad would be quite nutritious, and probably cost less than $1 each. If I skipped the beef and cheese, it would certainly cost less than $1 each.
The bulk of those meals would be rice and beans, and you can buy them in bulk, but they're still cheap even if you don't.
Had to upvote because you're entirely correct. Protein deficiency isn't really a thing in the developed world, except for those who are eating exceptionally poorly. There is one amino acid that is absent or very low in most legumes, but it can be found in most grains or nuts, so the slightest effort can remove the risk of deficiency.
Most nutritional diseases in the developed world are due to excess, not deficiency.