Chicken breast and white rice, doesn't get any cheaper then that. Assuming your trying to put on size, your gonna need way more calories then chicken and rice will provide tho. Peanut butter and jelly sandwiches are cheap and easy, you can buy powdered peanut butter if you want less fat.
Pasta is cheap, the high protein pastas are not terribly expensive either. If you're living in a dorm you should have a Foreman grill and a rice cooker, a mini fridge would be ultra helpful.
Cheap protein - chicken breast, certain cuts of pork, eggs, and you can usually find store brand cottage cheese and Greek yogurt for pretty cheap.
For cheap carbs you can't go wrong with white rice, pasta, potatoes and bananas. Store brand Rice Krispies is a super easily digestible carb source, but you can do any kind of cereal post-workout just mix it up with a protein shake instead of regular milk.
I wouldn't worry about fat sources, you'll be getting enough of them from your food.
A bonus mixing rice and beans together gives you a complete protein source. And if you need some vegetables, green peas, edamame, garbanzo beans and spinach are close to complete sources of protein. They're all missing something but they have pretty good amino acid profiles so you're getting like a double whammy of more extra protein with those vegetables selections. If you're really broke you can prioritize quantity over quality for sure but the fatter you get the harder you're going to have to work in the long run. So do your best to keep it somewhat clean
