Some useful and thoughtful responses here. My two cents:
-It's okay to vary your diet with the seasons, and also to not desperately try to get every nutrient into every meal. I'll have a lunch that's 90% corn on the cob in August (cost: maybe 50 cents?), or 70% ground pork after visiting the Chinese supermarket (cost: maybe 70 cents?).
-Some foods are just superstars in terms of filling you up, giving you nutrients, and being cheap. Dried legumes and grains, flours, in-season produce, carrots, onions, potatoes, cabbage. Buy big amounts, cook them yourself, add many flavour things, and make enough for later meals.
-Conversely, some foods offer little nutrition or flavour, but serve as some kind of symbolic "food" for people. Things like iceberg lettuce, or cucumbers, or out-of-season "fresh" tomatoes. You have my permission to not buy them.
-Almost every food culture has ingredients that cost more, but that make cheap food amazing. Things like Indian spices, parmesan, gochujang, dried mushrooms, fresh citrus, harissa. These are probably worth the investment.
-Almost everywhere is good for SOME kind of cost-cutting. The more rural you are, the more sense it makes to forage, grow your own, and direct some money or love toward neighbours who farm or hunt or fish. If you live in a big city, you must shop at large "ethnic" supermarkets.
-Almost all leftovers, trimmings, and other collateral damage can be turned into soup. Not just "oh crap, I have to eat soup to save money" soup, but good, good soup.
-Shopping well and making your own stuff does take some time, but once you get past a certain threshold, it stops being a chore and becomes a rewarding creative enterprise. I think we don't talk this up enough.