Google's click fraud guru Shuman Ghosemajumder says click fraud makes up less than 2% of all “invalid clicks”, which means the actual click fraud rate is more likely just a fraction of one percent. Andy Beal has the scoop over at Marketing Pilgrim.
Shuman is backing away from the 2-percent figure, saying only that "the total number of invalid clicks we detect – whether for suspected malicious or non-malicious intent – is in the single digit percentages. So third-party estimates which say that click fraud is 15% or higher appear to clearly be substantial exaggerations."
In another comment, he says there's no way to determine an exact click fraud rate:
"I did not provide a specific click fraud rate. We don't have such a metric to disclose, because there's no exact way to determine 'intent' and we certainly do cast the net wide in terms of throwing out many clicks which we know have nothing to do with fraud. The total percentage of all of those clicks that we don't charge advertisers for in this fashion is in the single digits."