Drew is selling math textbooks and using cost-per-click (CPC) bidding for his campaign. What’s the final cost each time his ad is clicked?
The projected maximum cost-per-click (max. CPC)
The average amount charged each time someone clicks on his ad
The average amount needed to make the ad appear somewhere on the page
The minimum amount needed to maintain a higher rank than the rank held by the next-highest bidder in the auction
Explanations:
Actual CPC is often less than max. CPC because with the AdWords auction, the most you’ll pay is what’s minimally required to hold your ad position and any ad formats shown with your ad, such as sitelinks. Keep in mind that your actual CPC may exceed your max. CPC if, for example, you’ve enabled Enhanced CPC or if you’ve set a bid adjustment.
Actual CPC is often less than max. CPC because with the AdWords auction, the most you’ll pay is what’s minimally required to hold your ad position and any ad formats shown with your ad, such as sitelinks. You pay just enough to beat the Ad Rank of the advertiser with the next highest Ad Rank (or the minimum Ad Rank necessary to appear in that position).
Read more here: https://support.google.com/adwords/answer/2464960