This question came up with both a client and at the last Websites 101, Part 3 (SEO) class:
Does Google Adwords charge you when someone clicks on your ad or when it appears on a search page (no click)?
It seems you can do it either way. On PPC Answers, the authors say
Your decision to use cost per click (CPC) bidding or cost per impression (CPM) bidding in Google AdWords depends entirely on your advertising objectives. CPC bidding is the most common type of bidding in Google AdWords because you only pay when your ad is clicked. CPM bidding is used when you’re only concerned about ad visibility and you aren’t worried about clicks or immediate conversions. Click here for the rest of the article…
For what Google has to say about costs for the two options, plus one additional, click this link: “How costs are calculated in AdWords.”