If 2020 has you pinching pennies, this site listing foods by calories per dollar may be helpful.
Engineer Michael Kirk says he was inspired to create the list after calculating cost per calorie of fast foods. He notes, "We think we know what food costs, but we have no idea what nutrition costs. We needed a way to value food."
The clear winner is wheat flour, which is also the winner in protein per dollar. If you'd rather not eat dry flour because it's a nice way to get salmonella, cheap white bread is not too bad for foods that require no prep. Slap some peanut butter on there and you have two top items in one delicious sandwich.