A maximum cost-per-acquisition (CPA) bid is the amount you're willing to pay for a conversion (such as a purchase or signup). When you enable the Conversion Optimizer for a campaign, you set a maximum CPA bid for each ad group.
A maximum CPA bid is not a target or average CPA bid. Its function is similar to that of a maximum CPC bid. If you set a maximum CPA bid of US$5, for example, the actual average CPA is likely to be somewhat less, such as US$3. Likewise, a maximum CPC bid of US$0.50 may result in an average CPC of US$0.30.
An important note
Your actual CPA depends on factors outside Google's control, so it may exceed the maximum CPA you specify. The Conversion Optimizer uses historical conversion data to predict the likelihood your ads will convert. However, your actual conversion rate can be affected by changes to your website and ads, or external factors such as increased competition. If your actual conversion rate turns out to be lower than the predicted conversion rate, your actual CPA may exceed your maximum CPA bid.
