>>144553
There is no "most optimal" protein, you gotta keep balance, and there are plenty of sources that are reasonably cheap depending on where you live, and what effort you put into cooking.
Some examples are:
>meat: pork and chicken breasts
>vegetables: peas, broccoli and other starch rich in minerals
>eggs and milk
>cereals and nuts
tl;dr, just fucking check the infographs and nutrition thread