Sous vide cooking. It’s easy to buy a bulk quantity of food, vac-pack and cook it, then freeze it. This saves time and money both on purchase, initial prep, and mealtime prep.
For example, we buy a whole, locally-grown, grass-fed chuck flap. We trim, bag, and cook the entire flap in one day. This provides my partner and me about six weeks of meals with high quality protein. Added bonus: the juice and gelatin in the bag after cooking makes excellent soup stock or cooking liquid for beans. Double added bonus: a sous vide chuck steak is just as good as the best ribeye fillet.
Also learn to use an entire chicken. For example, spatchcock and roast the chicken for dinner. Break down the carcass to get every scrap of meat. Make chicken salad the next day. Roast the bones, make a mirepoix, and make chicken stock. Use that to make chicken and dumplings or chicken soup. The two of us eat for a week from one chicken.
Learn about food preservation and safety: reusable containers, dangerous food conditions, fermentation, canning, making stocks… A huge part of saving money on food is not wasting any of it. Being able to buy in-season food when it’s cheaper and more nutritive is a big deal.
And on that note: avoid cheap, low-nutrition food. Sure, that industrial, NPK produce and ultra-processed box meal might be “affordable.” But those tend to be empty calories; you have to eat more of it to feel sated and get the nutrition you need. Locally grown, in-season foods tend to be better food values since you need to eat less of them to get the same micronutrients. See: “The Doritos Effect,” by Mark Schatzker.