^ It is more of a Lemonade Stand topic, but I'm guessing it might be inspired by the thread about rising food prices we had here recently; plus iron horse seldom posts in other forums.
I've cooked for multiple people on a tight food budget for most of my life...I don't really think in terms of individual cheap meals though; more in terms of a range of ingredients that are cheap, versatile and nutritious, then you loosely plan a week or so of meals at a time using those ingredients in multiple ways. At least here in the US, beans, eggs, and chicken are usually the cheapest protein staples; rice, pasta, potatoes and (if you bake) flour are usually the most economical starches. Then you round those out with cheap vegetables that keep well: onions, garlic, carrots, cabbage, celery, fresh or frozen broccoli and dark greens depending on season, frozen corn and peas, canned tomatoes and tomato products (save and freeze your cutting-board vegetable scraps for making stock). Then if you have a few dollars from your weekly food budget left over, spend it on slightly more expensive foods with flavors that go a long way: cheese, or canned fish, or cured meats, fresh herbs, etc. Spices and dried herbs are invaluable for staving off boredom, but if you get them at a supermarket, you'll mostly be paying for the jar or tin--make a trip a few times a year to an 'ethnic' market and buy them in plastic bags instead; that's usually also the place to find the cheapest prepared chile sauces, curry pastes, etc. From late spring to early fall, if you've got a local farmer's market, you'll probably be able to afford a wider variety of produce there; I like to oven-dry some tomatoes and make some roasted red bell pepper paste at that time, since those ingredients go a long way in flavoring things later, keep well, and are difficult to find affordably otherwise.
Dried beans are hands-down the cheapest base for multiple nutritious meals (stews, soups, pilafs, casseroles, burritos and enchiladas, etc.) and take well to all kinds of flavorings--lentils and split peas/dals are good curried, or cooked with tomato paste or red bell pepper paste; chickpeas are good curried, or cooked with garlic and herbs; black beans are good with Latin flavorings; blackeyed peas are good with chile pastes or Caribbean spices; kidney beans are good curried as well as in Tex-Mex chili; white beans are good with tomato sauce, garlic and herbs. Almost all beans combine well with vegetables, especially dark greens. Some of our favorites are: lentil minestrone with red bell pepper paste, olive oil and mint; cooked lentils and rice (or macaroni) lightly crisped with tomato paste and cumin, then topped with either browned onions and garlic, or with pickled onions; blackeyed peas sauteed with Caribbean seasonings (thyme, paprika, allspice, mustard) over rice; curried chickpea-vegetable stew with flatbread; black bean enchiladas; and various dal-vegetable curries over rice. Bean soups and stews are usually more flavorful if you saute the spices/herbs/garlic and onions separately then pour them in.
Eggs in spicy sauces are good over rice or breads (huevos rancheros or egg curry, for example). Italian/Spanish/Persian-style 'omelets,' which are dense and firm and use eggs more like a binder for potatoes, veggies or other ingredients, are filling and tasty. Pouring in a few whipped eggs near the end of cooking is a classic way to add protein and a more substantial feel to a dish that might otherwise seem painfully spartan--adding eggs, scallions, and a little ginger and soy sauce to humble chicken broth with rice gives you egg drop soup, or its Greek equivalent, avgolemono soup (whip the eggs with a little lemon juice, and toss in some dark greens too). Or pour the eggs over a simple vegetable dish such as sliced potatoes and greens baked with garlic and paprika for a nice basic casserole.
Chicken is usually the cheapest meat; buy it whole or bone-in and break it down yourself (save the bones and carcass for stock, which provides a flavor backbone for all kinds of dishes; save leftover scraps of the meat for enchiladas or fried rice). You can 'splurge' and serve it baked or fried as a main course in itself, or stretch it out by using smaller pieces to fill out stir-fries, pasta sauces, vegetable stews etc.
If you do your own baking, then you can have your own breads, flatbreads, cornbread and savory pancakes (not to mention baked breakfast and dessert treats) for super-cheap too, which makes for a nice alternative to rice, pasta and baked or sauteed potatoes. When I was in grad school, I used to love making dinner out of reheated flatbreads topped with whatever I had on hand--sliced hardboiled eggs, grated fresh tomatoes, and a 'salsa' of cilantro, garlic, chiles, cumin and cardamom was one of my favorites. There are numerous ways to put bread that's starting to go stale to good use in cooking, so you don't have to be cooking for a whole family to make it worth your while to bake bread.
This is really making me nostalgic for the weird mix of Southern country and Greek peasant cooking I grew up on.