Spaghetti is a good fallback for me - a can of tomato paste, can of tomatoes, can of mushrooms, and a whack of ground beef and spices will keep me fed for a while and is cheap and easy to put together.
Also simple smothered chicken - some chicken thighs (boneless or skin removed), browned with flour/salt/pepper in a frying pan then tossed into a dutch oven for an hour and a half at low temperatures (or in a slow cooker for 8). Cover it with chopped carrot, onion, and celery, and pour in enough chicken broth to cover the chicken. Serve on rice.
Actually, slow cooker stuff in general is great (and cheap). My spaghetti sauce goes well in it. Same with chili (almost identical ingredients to spaghetti sauce other than beans and spices), stews, and soups.
Soups are particularly easy because they're very forgiving of leftovers. Toss in some slightly stale rice, broth (hell, just use powdered if you're really cheap), leftover chopped chicken, frozen mixed vegetables, potatoes, carrots, onion, whatever you've got in your fridge that would work.
I also like to buy up packs of ground beef and brown it up - keeps for a few days and is good sprinkled on salads, mixed into soups or sauces or for making tacos.
... But scrambled eggs with hot sauce or jam is just awesome and cheap
