Metrics
CPC (cost per click)
CPC is the average cost paid for each click, the dominant pricing model in search and common across social. It prices interest rather than delivery, but a click is still an intermediate step - CPC only matters in combination with what happens after the click.
In practice
A cheap click that never converts is more expensive than a dear one that does. CPC belongs alongside conversion rate and CPA, not in place of them.
Rising CPCs signal auction pressure: more competitors, higher bids or declining quality signals. Diagnose before simply paying more.