If your shop takes $3600 an hour in revenue, but there's a problem with the till which means that people can't pay for 10 seconds, you haven't lost $10 in revenue, you've just shifted revenue from $1/second to $0/second for 10 seconds and $2/second for the next 10 seconds.
Yup, the only "real" cost there is a customer who decides not to buy after all, or buys elsewhere instead. But that's pretty unlikely, especially for short outages. And it's even less of an issue for entities with a lot of stickiness like social networks (Facebook, Twitter) or shopping websites with robust loyalty programs (Amazon Prime).
