If you can get a meal with fiber and protein in it, you're golden.
Frozen vegetables are ideal in terms of cost and ease of use (if you have a freezer and someplace to heat them up). Things like whole wheat bread, bagels, tortillas, pitas, etc are relatively cheap and go a long way to making a filling meal. Just look for ones that have some fiber in them.
On the protein front, beans are full of fiber and protein, keep forever with no refrigeration, and they're really cheap (this assumes you have the facilities to cook them, though).
Other cheap and healthy proteins: eggs and store brand tofu.
Tofu can be baked or crumbled into a salad, AND it can be frozen, so you can stock up when it's cheap. There's also shelf stable varieties, so if you don't have a fridge that could be a life saver.
Eggs can be hard boiled for salads, or just fried up and eaten in a sandwich or burrito. They keep for a pretty long time, too.
If you're eating from a dinning hall, just look up appropriate portion sizes for foods (ie, one serving of meat should be the size of your palm, starches should be about the size of a tennis ball, etc) so you can estimate calories. Go for any combo of protein and fiber.
For exercise, there's plenty you can do at home without equipment! For a bunch of free exercise videos, try
http://www.youtube.com/user/BeFit
Walking is also a great calorie burner, believe it or not, because most people are fit enough to do quite a bit of it.