I'm a vegetarian, so my food (noodles, beans, rice, veggies) is pretty cheap anyway. But my honey eats meat, so I've had to learn what to feed...those people.
Sometimes, I get him a big hunk of meat, some cheap cut that would normally be tough and stringy. I put it in the crockpot on low, for like 12 hours or so. Throw in some celery, garlic, onions, and a couple of glugs of balsamic vinegar. Well, you could use wine instead, but that would be wasteful, right? It's important to not add any salt while it cooks, as salt will make it tough. The acid in the vinegar or wine acts as a tenderizer. After it's cooked all day, it will fall apart as you slice it, it's very tender. My man says it's delicious, but I'm not a barbarian eater of flesh like he is, so I wouldn't know for sure. I serve it with some sort of noodle or rice dish, and whatever veggies look good. There's tons left over for his lunches.
When I was a student, I ate a lot of cereal. It's cheap, yummy, quick to prepare, and very nutritious. And, there's always delightful, delicious ramen; add some veggies and a handful of grated cheese, and it's a little bit nutritious.