I think that just looking at the cost of a particular system isn't taking a complete view of competition or the market.
So what I'd suggest is that there's perhaps a cost to accepting cards, as there is to accepting any form of payment like cash, but credit cards deliver tremendous value to both retailers and consumers. To consumers, they deliver convenience and speed. They allow consumers to have purchasing power in their wallet rather than rely on the amount of cash they're carrying around at a given point in time. They also have purchase protection features that benefit consumers. There are various charge-back rights, and there are other sorts of protection measures and insurances that come with cards and benefit consumers.
On the retailers' side, there's obviously the benefit of creating satisfied customers. We have many studies showing that consumers spend more at retailers when they're using their cards versus cash. It's a way for retailers to extend instant credit to their consumers, which allows smaller retailers who wouldn't be able to afford a credit program to compete more effectively with large retailers.
The final point I'd make is that it's a form of guaranteed payment. Retailers don't have to worry about fraudulent money; they don't have to worry about bounced cheques. They are guaranteed those funds, and guaranteed them generally within about two business days.