Ad buys are calculated by the types of purchase models that are often associated with specific ad types.
Pay-per-click (PPC) ads are charged by the number of times a user clicks on the ad; Cost per Action or Acquisition (CPA) ads are charged by the number of these defined actions that occur; Cost per Mille (CPM) ads are charged for every 1000 displays or impressions.
For most digital ads, the cost for each ad click (or CPA or CPM) varies greatly by the ad context and competition or popularity—the keyword, subject, display location, etc. And in many cases advertisers bid in advance with the highest value they are willing to pay for a particular ad in a particular location, e.g., on Google’s search engine results page (SERP), on Google’s Display Network, or on specific content-defined websites.
Learn more about digital advertising here.