I have a price book where I compare the items I buy at cost per ounce at various stores. The items that are by far cheaper at costco, all organic unless I say otherwise: brown rice, quinoa, chicken, ground beef, frozen berries, frozen corn, frozen peas, orange juice (conventional), honey, bullion, olive oil, canned tomato sauce, canned tomato paste. The milk and cheese is cheaper than regular price at other store, but they beat costco with good sales, same with eggs. Dog food and kitty litter is way cheaper at costco. A lot of household stuff, like trashbags and LED lightbulbs, are also way cheaper per unit than other sources.
We have the executive membership and to costco credit card. I take the amount of the membership out of these rewards every year, and then anything else is bonus. With these rewards our membership is basically free, and then we are saving a lot.