A recent study by ClickForensics showed the amount of click fraud to be 13.7%. I think this number is too low because it doesn't take into account the types of click fraud that are difficult to detect due to adware, click rings, compromised machines, etc.
As has been discussed here earlier, the amount of click fraud on higher tier ad networks was considerably higher than on lower tiers.
If they don't detail this figure and how was it obtained, it's just advertising. I can say it's 50%, and it could be true (for certain SE could be even more..), so without details it's absolutely useless. When talking about figures, every aspect matters. Do you have a source, a detailed article?