Well, if we make that a metric, it'll be gamed, no? Don't track that at all. The advertisement should run for a set amount of time, and the correct metric to track is successful leads that originate from the campaign, which is easy to measure if you're tracking leads (you really should be) and the campaign has a unique landing page.

Don't bother to track ad clicks (because they can be gamed) and don't sell ads based on clicks (because then they WILL be gamed) and the problem vanishes. Click fraud is a problem the advertising industry created for itself, and other advertising mediums without invasive user-tracking seem to get on just fine without it.

Some kind of statistical analysis of online clicks versus uptick in actual sales in, and slightly beyond, the time window of the advertising campaign.

Base payment scales on impartial ratings of website popularity.

I'm harshly anti-ads (for a swathe of reasons), but the problems of ad-fraud is an internet-unique problem. Advertising campaigns on TV, Radio, and paper mediums didn't track views (clicks didn't exist, my gosh!). An assumed value was given to the advertising based on the popularity of the particular TV show, radio program, magazine type, etc. AND the ads were generally targeted to the audience of the medium.

The advertising industry had this problem solved before it even became a problem. They wanted more, and are whinging about people blocking their methods to gain this 'more'.

Just don't create online dossiers on individuals, don't provide yet another vector for malware, don't invade our spaces with uninvited sound, don't slow down the sites I've chosen to view, don't waste my bandwidth.

CPA (cost per acquisition) seemed like it was gaining steam over CPC (cost per click) and CPM (cost per thousand views) when I left Yahoo! in 2011.

Did that not go anywhere?

Vast majority still backs into a CPM. CPC/CPA is mostly by the affiliate/performance advertisers.

It's still a vanity-metric CPC game :(