I’m told the right percentage of click fraud is about 15% (or 800 million dollars a year) by San Francisco Chronicle - got the news from MarketingShift.

"Almost 15 percent of all ad clicks are fraudulent, according to a new survey from market research firm Outsell.com

Advertisers are becoming more aware of the problem as 75 percent believe that they overpaid for clicks, and 37 percent said they have or would reduce their ad spending because of click fraud.

If numbers are correct then the class-action lawsuit only covers a fraction of the actual costs incurred by Advertisers.