Earlier quoted context omitted.
That cheapness is mainly due to non-priced-in externalities (greenhouse gas farts! water use!) and poor treatment of animals (not that I'm a PETA member or anything). EDIT: And as mentioned elsewhere: government subsidies in many countries (and/or import/export controls).
So you agree that there are meats which are cheap sources of protein?
There is increasing pressure to lower farming subsides in most of the world in part because food costs have already fallen so much. 1935 1/lb chicken $5.07 (inflation adj), 2011 1lb chicken: $1.34 But, also from a wide range of more politically charged issues.