Black beans cooked with onions sauteed in bacon fat and seasoned with Goya Sazon. Brown rice.
Spinach cooked with garlic, olive oil, and chile flakes.
Canned mackerel is ass-nasty but at 75 cents for a 15 oz jar at the dollar store, if you can figure out a way to cook it (hint... Will it blend? Remove from can first... hahahahaha), it's the cheapest source of animal protein I can think of. Spicy mustard cuts the taste of oily fish well.
Soy protein in all its forms, from mock-dairy products to tofu to textured soy protein (stellar in chili), is cheap and good for ya.
Also, check out the crash-n-dent grocery stores that sell what I refer to as 'used food'- vegetables too ugly for the supermarket, packaged food with damaged packages, and meat that is less than 48 hours from its sell-by date.
This po' B gets down with the used food store fo sho.

A final word of advice, look to the cuisines of developing countries for ways to eat cheap. Don't look at individual DISHES, look at the whole diet.

I'd start with India, where even the vegetarian cuisine is extensive and HIGHLY nutritious. Yoghurt- based drinks, sauces, and desserts are common and paired with beans and rice or bread give you what is considered a 'complete' protein, meaning that all the amino acids are present and your body will process it most efficiently. Also some of the spices in Indian food such as ginger, garlic, asafaeotida, turmeric, and a couple others are considered essential not only for their flavor but for their digestive or other healthy qualities.