The PayPal affiliate program isn't to get people to sign up to pay for items- it's to refer merchants to PayPal. The bounty on doing so is high. For each online merchant that you convince to accept PayPal you can receive up to $1K. Money is made by the affilaite in the form of .5 percent of each of the transactions that goes through PayPal for one year, with a maximum of $1K.
PayPal provides a banner ad as well as a simple link that can be used to refer merchants. There's a lot of potential in this program, and it's going to be the focus of the next PLR article package on The PLR. I'm currently putting together a few articles about why merchants should use PayPal and how it benefits them over other payment methods. Using some of these articles on a small site about PayPal, or on a Squidoo lens or HubPage, or even on one page of an existing website, has the potential to bring in a tidy residual income.