The problem is that a lot of those sources of protein aren’t just protein. Take legumes; they all have a ton of carbs as well. Nuts? Loaded with fats. (Same with pork belly, TBH; that’s a very fatty cut.) On the other hand, boneless, skinless chicken breast has a trivial amount of fat and carbs.
If you were plotting a 3-dimensional chart, factoring in proteins, cost, and other macronutrients, you would likely find that boneless, skinless chicken in general was the highest combination of both purity of protein, and price.
The problem is that a lot of those sources of protein aren’t just protein. Take legumes; they all have a ton of carbs as well. Nuts? Loaded with fats. (Same with pork belly, TBH; that’s a very fatty cut.) On the other hand, boneless, skinless chicken breast has a trivial amount of fat and carbs.
If you were plotting a 3-dimensional chart, factoring in proteins, cost, and other macronutrients, you would likely find that boneless, skinless chicken in general was the highest combination of both purity of protein, and price.