Beans are a poor shopper's best friend. The cheapest source of protein, fiber, vitamins...they are the greatest.
Make a big pot of chili with dried or canned beans, tomatoes, tomato paste, onions, garlic, chili powder, cumin, maybe some bell pepper. You could add TVP if you like. All of that is very cheap if you have the spices on hand. Serve with brown rice, also cheap.
Dried lentils are totally cheap. Make a pot of lentil soup with green/brown lentils, onions, celery, carrots, a bay leaf or two if you have them, and some veggie stock or a veggie boullion cube. Lots of black pepper and some salt.
Potatoes are incredibly cheap. Have baked potatoes with baked beans or refried beans and salsa.
Do a chickpea-tomato sauce for pasta. Pasta is nice and cheap, and the bean sauce is more nutrient-rich than marinara. PM me if you'd like my recipe for the sauce. Peanut butter pasta is yummy, too--I'm sure someone here has a recipe.
Pasta e fagioli is cheap and deelish--saute a couple onions and some garlic, add some broth or water and a couple cans of tomatoes, a couple teaspoons of oregano and basil, cook it down, add three cans of various beans, simmer until they're soft, and mix with a pound of cooked pasta. You'll be eating it for a week.
Try to get some fruit in there too! Of course bananas are the cheapest. Bananas and PB would make a cheap snack.