Banner ads are usually sold by the page impression - each time a user views one Web page is counted as one impression. Prices are quoted in "CPM", a holdover from print ad days which means "cost per thousand". For example, if the CPM for a particular campaign is $20, then it would cost $2,000 for 100,000 impressions.

Banner ads yield two benefits. First, they may entice users to click on the banner and go to the advertiser's site. Each time this happens is called a "clickthrough", and the number of clickthroughs divided by the number of impressions is called the "clickthrough ratio" or "click rate". This is easy to measure, and provides a direct measurement of a banner's effectiveness.

The second benefit of ad banners is building brand recognition. Despite ad agency claims to the contrary, this is almost impossible to measure. User surveys and market research can provide clues, but they are very imprecise and conjectural compared to the straightforward business of measuring clickthroughs and site traffic. Whether brand recognition is part of your strategy depends entirely upon your individual situation. If it is, then things like click ratios and conversion rates can't be used as absolute criteria, and the process of measuring effectiveness becomes much more subjective.