What is masthead billing based on?
- A flat daily fee
- Impressions and clicks
As an advertiser or ad agency, you can buy placements on a reservation basis–instead of through the AdWords auction–when you want to pay based on the number of impressions (also known as cost-per-thousand impressions) or per day (cost-per-day). Reservation campaigns also let you buy impressions at a fixed rate.
Read more here: https://support.google.com/partners/answer/6030919?hl=en