Informative post from Aneace on the relative size of interchange, being the method banks make money from merchants on credit card purchases. There is interchange on debit cards, in addition to these numbers.

Aneace's Blog: The interchange industry is bigger than …

How big is the payments industry if you leave out loan revenue and simply look at fees paid by merchants and consumers for the convenience of using plastic? The bulk of that revenue consists of interchange fees paid by merchants to card issuers.

He goes on to point out the relativity of size for interchange.

Much bigger than the electronic game industry ($28.5 billion worldwide).Much bigger than Hollywood box office sales ($23 billion worldwide).

Bigger even than the music industry ($30 to $35 billion worldwide).

Bigger than venture capital investments worldwide ($31 billion).

Bigger than the microprocessor industry ($35 billion worldwide).

And almost as big as biotech ($48 billion in the US).