What you’re describing is Cost Per Click. While still in use with some advertisers, most have shifted towards Cost Per Impression. CPI has a much lower cost per impression, (typically paid per thousand impressions) but it usually balances out because you’re not wholly reliant on clicks to generate revenue. If only 1/1000 people actually click the ad to begin with, then the cost will be the same.

The benefit to CPI is that you’re able to spread your advertising much farther, since you’re able to essentially purchase a thousand ad slots at a time.