According to the most recent click fraud rate from Google - click fraud is only 0.002% of all clicks. Danny Sullivan has an in-depth analysis of the topic, emphasizing on the importance of detected click fraud rates :
"Finally, we have a click fraud rate from Google itself: less than 0.02 percent of all clicks slip past its filters and are caught after advertisers request reviews. That low figure is sure to bring out the critics who will disagree. Below, more about how Google comes up with the figure plus some click fraud fighting initiatives it plans to implement later this year.Why release this figure now, when many have wanted it for literally years?
"We've been working to be more transparent and informative on the issues related to click fraud. Recently, this metric has been something advertisers have specifically asked for and we agree that is useful in describing the scope of the problem. Further, it is something we measure and use to monitor the performance of our click fraud detection systems," said Shuman Ghosemajumder, business product manager for trust & safety at Google."
Stats courtesy of Clickfraudindex who by the way started blogging recently.