I second beans, and add potatoes to that list. Your "normal" potato and sweet potato or yam are all very nutritious and cheap. They can also be prepared in a wide variety of ways or used to bulk up more expensive ingredients like meat. Mashed potatoes are a favorite of mine, they can have all kinds of things added to them or be used to top shepherds pie which is another cheap, easy and nutritious dish. Cabbage is another good one. Eggs are cheap. A dollar fifty for a dozen eggs is a week's worth of two egg omelets at 25 cents each. Add a sprinkle of cheese, a bit of butter and a sprinkle of salt and pepper and you're looking at breakfast for less then fifty cents. Eggs can also be turned into a fancy sauce for a mundane meal. Like topping microwaved, frozen broccoli with hollandaise or bernaise sauce. Anyway you eat them, they're a good source of protein and other nutrition.

Your spice rack is also your best friend. Add a few spices to things can add huge flavor with relatively little impact on your wallet. A few of my favorites are mustard powder, chipolte, black pepper and paprika.

Like scafool said, if you buy it pre-made, learn to make it yourself. You can make almost everything you buy in a box yourself for much cheaper, and as a bonus it will be healthier and taste better. Take hot chocolate, make it yourself it contains cocoa powder, sugar and milk. Well... and optional salt, vanilla extract, chili powder, nutmeg or cinnamon, whatever you want. You can even decide what fat content you want in your milk. Buy it pre-made, it has an ingredient list two inches long in fine print. Mostly names only our chemist friend Blast could love. Likewise, the fancier the name on the package, the higher the profit margin. A great book for learning how simple these "fancy" dishes are to make is Cooking for Dummies. You may not want people to see it on your shelf, but they'll love the food you cook from it.

Finally, turn cooking into entertainment. If its a chore, you aren't going to want to spend the time to save the money. Cooking is how I decompress in the evenings. I don't have a significant other right now, but when I was in college, my girlfriend and I would cook together in the evenings. It gave us a chance to spend time together, talk about our days and share our schedules so we could plan events together. If you're like me and alone, a gourmet meal and half an hour of TV beats a microwave meal and two hours of TV any day. Plus, it gives you an excuse to invite someone over.