cheapest would be trying to use whatever you might already on hand. yummly has tools to plug in what you've got and come up with recipes.
EASY cheapish
-taco soup (ground beef, bell pepper, onion, 1 can each corn (optional), black beans, pinto beans, kidney beans, 2 cans diced tomatoes, 1 oz dry ranch dressing mix/1 oz taco seasoning,
-spanish rice (ground beef, white rice, salsa, oil),
-pulled pork (pork is the least expensive meat around here these days)-pork (cheapest cut b/c it will be shredded anyway), bbq sauce, buns,
-baked potato soup (potatos, milk (or cream), onion, broth (dry cubes are least expensive), butter). if desired-sour cream, cheese, bacon bits, chives for topping.