American Express would only let go of the Costco business when there was no way to make a profit at the numbers Citi was talking about. It’s the perfect example of winner’s curse. Everyone knows what the business is worth, the one who wins the auction is the one that overpays.
Citi got a huge portfolio — over 11 million American Express cards across more than 7 million accounts. But they didn’t get the same card exclusivity Amex had, Costco will accept any Visa — even those of competitors.