>>76247343
Absolute cheapest:
Whole chicken, frozen
Pork shoulder
Beef, Top round or London broil or ground beef
Whole milk (not for a cut obv)

Convenient:
Whey
Eggs
Greek yogurt
Tuna

So for example
Eggs + Lentils or beans and Rice
Milk + Peanut Butter Sandwich
Canned Tuna + Rice + Frozen Veggies
Bulk Pork Shoulder Roast + Potatoes
Whey + Greek yogurt or milk