I'm sure it varies from area to area but here in the Seattle area I've found that as compared to the standard supermarkets (meaning Safeway, QFC, Fred Meyer), Costco sells prime grade beef at the same price as supermarkets sell choice grade and organic chicken at the same price as supermarkets sell conventional. But what really gives Costco's meat department the edge is that their meat looks so much better/fresher than the supermarkets.

It's a similar story for organic produce, eggs, and so on. Items like cheese, dried fruit and nuts are about half the price of comparable quality elsewhere. And then there's items like olive oil where Costco sells at a great price with the added bonus that you get actual olive oil and not some blend of unknown origin.

But if you live near enough to a Costco and own a car, the real killer-app is their gasoline. In my area, it is 50-60 cents a gallon cheaper than gas stations. From this alone my membership would more than pay for itself even if I never stepped inside the store.