For meat and fish: Supermarkets tend to have a pound or two of chicken and other meats at reasonable prices which equates to 1-3 meals. A chicken breast (or a few thighs) easily holds up for a week or so in the fridge as long as you put it in a sealed container.

And you can save a lot more money if you buy a larger pack, break it up into "meals", bag them, and freeze them. I am lazy and just do plastic-wrap -> freezer bag and give it the sniff test before cooking but smarter people use a more rigorous system.