In short
Cost per click is the price paid for each click on an ad. It is a useful input metric, good for diagnosing auction dynamics and spotting when competition or ad quality is moving your costs, and a poor optimization target on its own. The reason is simple: a cheap click that never converts is expensive, and a costlier click that produces a valuable customer is cheap. Optimizing to minimize CPC in isolation tends to buy traffic that looks efficient and performs badly. CPC belongs beside conversion and value data, where it helps explain results rather than define them. Read alongside conversion rate and customer value, a rising CPC can be fine if the traffic converts well, and a falling CPC can be a warning if quality is dropping with it. Treat CPC as a diagnostic gauge on the dashboard rather than the number you steer by, and it earns its place in analysis instead of distorting it.
What CPC is good for
Diagnosing auction pressure, creative fatigue, and audience saturation. A rising CPC with stable conversion rates is a market signal; a falling CPC with collapsing conversion rates is a quality problem.
Why it fails as a target
Optimizing to CPC buys the cheapest clicks available, and the cheapest clicks skew toward users unlikely to convert. Judge campaigns at cost per conversion or cost per qualified outcome, with CPC as a diagnostic underneath.
Why is the cheapest CPC often the wrong goal?
A campaign celebrated for low CPC turns out to drive few sales, while a higher-CPC campaign quietly produces the revenue. Judged on cost per acquired customer rather than cost per click, the ranking flips.
Reference: Google Ads Help, cost-per-click (CPC)