Advertising
What is Cost Per Click?
CPC
What you actually pay each time someone clicks your ad. It is the price of traffic, and it moves with competition, so a rising CPC can wreck your efficiency even when nothing about your own campaign has changed.
Why it matters in practice
CPC is set by the auction, not by you: your bid is the ceiling, and what clears is usually below it because it depends on what everyone else bid and on how relevant your ad looks. It matters because ACoS is really three things multiplied together, and CPC is the one you control least, so an efficiency problem is often a traffic price problem wearing a campaign costume. In practice CPC climbs into a category's peak season and settles afterwards, which is why the same campaign at the same bid can look efficient in one quarter and wasteful in the next. The useful habit is to read CPC beside conversion rate: paying more per click is fine if the click is worth more, and only a problem when it is not.
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