Visa earns data processing revenues from the authorization, clearing, settlement, transaction processing services and other maintenance and support services that facilitate transaction and information processing among customers globally and Visa Europe.

FORBES: Three Businesses Make Visa Worth $86

Instead of charging a fixed transaction fee for clearing the rights, Corbis tries to buy low and sell high, capturing the spread as revenue.

FORBES: Image Enhancement

Furthermore, processing costs are low: about 30 cents per transaction, half the cost of clearing a cheque.

ECONOMIST: Debit cards