Suppose a keyword research tool that requires a monthly fee has an affiliate program that pays affiliates 50% of the monthly fee for all sales they generate. You wouldn’t get just one commission check – you’d get a commission check for every month the customer paid his monthly fee. Make one sale – get a check every month. If these products also happen to have a high price AND a high commission rate then one sale alone could bring in $50 or more every month. Just ten sales and you've more than paid your car payment, utility bills, or acquired some extra spending cash for months at a time.