Importance of Cost per Mile in detail In marketing, cost per mile (CPM) is a key metric used to determine the cost effectiveness of an advertising campaign. CPM is calculated by dividing the cost of the ad campaign by the number of impressions (in thousands). The resulting number allows marketers to compare the cost of different ad campaigns and determine which is the most cost effective. CPM is a useful metric for both online and offline advertising, and can be used to compare the cost of different media, such as television, radio, print, and digital. CPM payment method is also preferred when a brand wants to run a retargeting ads campaign.