I don't know if this is how Costco itself thinks of it, but you can get to a pretty decent understanding of their financials by assuming they sell everything at

I don't know if this is how Costco itself thinks of it, but you can get to a pretty decent understanding of their financials by assuming they sell everything at cost and pocket the membership fees as profit. Their actual profits tend to be about 50% higher than that implies, but given the size of the rest of the business that means the membership fees are only about 2% of the revenue overall it's not a bad approximation. They can twiddle with dropping a bit of price here and raising it somewhere else and still have the same ratio of cost. Of course you can still think of things in terms of the cost per item versus the profit per item, but the stability of that ratio over many years makes me wonder if maybe Costco thinks of the membership fees that way.