Budget cooking has a reputation problem. It gets associated with sad sandwiches, plain rice, and food that technically keeps you alive. The reality: some of the best-tasting and most nutritionally dense ingredients are also the cheapest. The gap between expensive cooking and cheap cooking is mostly about technique, not ingredients.
These are the highest-value ingredients by cost-per-gram of protein, calories, or both:
Leek, potato and lentil soup: 180 kcal per serving, 9g protein, 4 servings from cheap vegetables and lentils. Probably under $1.20 per serving at current prices. Freezes well.
Mushroom and onion scrambled eggs: 3 eggs plus a handful of mushrooms. Under $1.50 total for a satisfying meal at around 220 kcal, 16g protein.
Lemon-infused cabbage salad: half a head of cabbage, lemon, olive oil. Side dish for 4 people for under $1 total.
Five-minute fried sardines with olives: a can of sardines, a handful of olives, 5 minutes. Sounds unimpressive. Tastes surprisingly good, and the macros are excellent - high protein, high omega-3, low cost.
Pan-roasted chicken thighs: bone-in thighs are cheap, and this recipe requires no special ingredients. ~250 kcal, 28g protein, well under $3 per serving in most markets.
Balsamic chicken and mushrooms: chicken plus mushrooms plus balsamic vinegar (a bottle of which lasts months). ~280 kcal, 38g protein. Tastes restaurant-quality, costs under $4 per serving.
The most common budget cooking mistakes:
Build this once and maintain it. It covers most meals:
Dried lentils, canned tomatoes, canned beans, canned fish, eggs, olive oil, onions, garlic, a good spice set (cumin, paprika, coriander, chili flakes), soy sauce or fish sauce, vinegar, dried pasta or rice. Most of this is shelf-stable and doesn't spoil. Once you have the pantry, your variable grocery cost drops significantly.
For more on reducing cooking friction overall, see why you hate cooking and how to fix it.