There’s a few things I usually have at home because they’re cheap, can be used for various dishes with or without additional ingredients and I will actually eat them before they spoil:

Beans, lentils, tomato paste, eggs, peanuts, cottage cheese, smoked tofu (not neccessarily a cheap item but I only use half a block or less per dish), bread, rice, spring onions, bell pepper, frozen spinach, hummus, cucumber.