Oatmeal is pretty cheap. One of the big cartons usually costs like 2 bucks and if you eat that for breakfast everyday it will last you a week. What I do is heat up some water with a stick of cinnamon until it boils. Once it's boiling I put the heat on medium for a a couple more minutes and add raisins and banana to it.
Beans and Rice is cheap too and will provide as much protein as more expensive meat. Black beans and brown rice are the best I think. I boil the beans with a piece of garlic and a few dashes of salt. The rice I just boil it with water. You can get beans and rice for a dollar a pound and sometimes two pounds for a dollar. A pound usually lasts me a week.
Lentils are just as good as beans in price/taste and easy to make. I like to boil these with garlic, onion, cilantro and tomatoes.
Frozen vegetables that come in a bag are also cheap. I like to get that mixed bag that comes with corn, peas, carrots, and green beans. Usually costs about a dollar per bag. I mix this with my rice.
Corn tortillas go great with beans and rice. Not sure if corn flour is bad for you but I can't stop eating these. A bag of these should cost you about 1-3 bucks and last a week.
Finally green chili is awesome.
Goes great with beans/beef and tortillas. Here's the recipe: