Here is a list of foods that are typically affordable:
Macaroni and cheese - Some discount stores sell this for 3 for $1.00: Bring a durable microwavable cup or bowl.
Potatoes - Buying these in bulk, a 10 pound bag may cost only $3.49: Cook at home, store in cold lunch bag, pop in microwave at college; you can also add water instead of milk if you still enjoy the taste
Ramen noodles - $0.20 - $0.40 per pack and $0.33 to $0.65 per cup a soup: Just add water, heat in microwave, and serve
Rice - Buy in bulk 20lb bag for as low as $15.99: Cook at home, reheat at the college campus
Apples/oranges/pears - sometimes you can find a 5 pound bag of apples/oranges for $2.50 - $2.99
Bananas - can be around $0.40 per pound on sale: Keep in mind that these tend to have a short shelf life
Spaghetti - Some discount stores sell the noodles for only $0.75 and 1 lb of the sauce starts at $2.00 - buying this in a 5 pound can usually starts at $4.99: Cook this at home, store in fridge in an air sealed container to extend shelf life for leftovers. Bring in cold lunch bag and re-heat in microwave.
Chicken - 10 pound bag of legs for as low as $4.99-$6.99 on sale: Heat at home, bring in cold lunch bag to school, reheat in microwave.