Cost per acquisition (CPA), also known as cost per action, is a metric that measures your advertising cost by a conversion, as opposed to a click. This can include downloading content, filling a form, or making a purchase. Generally, CPA will be higher than CPC as not everyone who clicks your ad will go on to complete.
With this type of advertising, you will pay an agreed fee for each desired action.
Cost per action can be calculated by dividing your total revenue generated by an ad campaign, or, the associated value of an action, by the total number of actions on that specific advert. E.g. £1,000 of revenue is generated from a pay-to-buy advert where a total of 39 people completed the action = £25.60