What make you think so......is there any solid reason for your belief that it cannot be such a huge number I feel it is possible with all those small publishers...trying to earn just quick bucks The source of information is clickforensics
Adwords advertisers can easily avoid most of click fraud if they choose to display their ads on search network ONLY. If they do decide to show ads on content network also, they should choose an industry that is not targeted by MFA webmasters. So which niches do MFA scammers choose? Look at the high paying keyword lists and you will get the idea where you find them. Or you could separate your bids by content and search. You leave your search network bids higher 0.3$ - 1$, but set a separate price for the content network, a reasonable rate would be 0.05$ - 0.10$.