I don’t keep an official one, but I do know the prices in my head. I also have items I won’t buy until they get low enough on sale, sort of like a “cut off” amount on what I’ll pay, that I also have in my head.
I would start with making a list of the items you buy every time you shop- milk, eggs, bread, meats, etc. Anything you put into the cart every time should be on the price list. Mark the actual price, not the sale price. Mark the price from any stores you regularly go to, so you know which store is cheaper.