Eggs, rice, beans, pasta, oatmeal get some spices and you'll be all set for three meals. I've been poor as heck, so I've been there. As Triumph said you can by rice in bulk for super cheap, same for oatmeal. Powdered milk is also cheap and provides good protein.
Ramen ain't bad if you dump the flavor packet.
A staple of mine in college was ramen noodles w/o flavor packet, a peanut butter sauce made with natural peanut butter, soy sauce and fish sauce, topped with a fried egg and garnished with sliced scallions.