As people have already pointed out, a lot of cheap foods are mostly grains, but you can add meats and vegetables to make them more well-rounded meals. When I wasn't working last year and didn't have any money, I ate a lot of rice or noodles with chopped veggies and a little bit of chicken.
Rice-A-Roni was good for this. At about $1 a box, it made for two meals with veggies and chicken added. Good old spaghetti is cheap too, and you can make a cheap simple sauce with canned tomatoes, tomato paste, onions, olive oil, garlic, salt and pepper. Some other nutritious and cheap foods are beans, canned vegetables, tortillas and eggs. Hot dogs can be pretty cheap, and while not exactly nutritious, they can be okay if you eat them with healthier foods.
Everybody have fun tonight. Everybody Wang Chung tonight.