From Wikipedia:

In the flat-rate model, the advertiser and publisher agree upon a fixed amount that will be paid for each click. In many cases the publisher has a rate card that lists the Cost Per Click (CPC) within different areas of their website or network. These various amounts are often related to the content on pages, with content that generally attracts more valuable visitors having a higher CPC than content that attracts less valuable visitors. However, in many cases advertisers can negotiate lower rates, especially when committing to a long-term or high-value contract.

The flat-rate model is particularly common to comparison shopping engines, which typically publish rate cards. However, these rates are sometimes minimal, and advertisers can pay more for greater visibility. These sites are usually neatly compartmentalized into product or service categories, allowing a high degree of targeting by advertisers. In many cases, the entire core content of these sites is paid ads.