DrPizza
Administrator Elite Member Goat Whisperer
Just to play devils advocate, I don't think the relatively few computers line is true. For starters, the scale is tremendous - VISA alone processed 28 billion transactions in 2006, that averages out to ~900 transactions per second. And if you consider uneven distribution it probably spikes to 10,000 transactions per second. You need quite a bit more than a few computers to process those (especially since you need to keep audit record of every transactions).
Secondly, they're probably other expenses, like Fraud, customer support, etc - although I'm not sure how many of those are paid by networks, vs issuers.
You did see that word in front of "few," didn't you? Here's a hint - it starts with an "r" and rhymes with selatively. I didn't say they didn't have other expenses, but they make a shit ton of profit for doing relatively little.