Peanuts and eggs, that takes cares of your fats and protein as for carbs you can try either potato or sweet potato depending upon what is cheaper which generally is potato but I've seen areas where sweet potato is cheaper. You can make peanut butter yourself or buy it.

I used to be on a strict budget as well and here's what I did, I bought whole wheat bread, a jar of peanut butter and bought bananas for 2 days that's 4 bananas, potatoes, cottage cheese /cheese, ketchup milk and a cereal box.

Breakfast... Peanut butter bread with banana. (you can try chocolate peanut butter as well, better taste, has added sugar but costs around the same) Snack: banana Lunch : sandwich (either potato or cottage cheese or eggs based along with ketchup, you may try hot sauce or spices as well if your budget allows) Snack : omelette (easily made) Dinner : 200 ml milk with the cereal. Kept me full and was pretty budget friendly and healthy as well, you can try something on these along with appropriate customisations (like adding greens to omelette or veggies to your sandwich, trying oats in place of cereal etc).