Sarah at the Inside AdWords crew has posted 'Googles Overall Numbers' on click fraud.

"As part of our effort to provide you with more information on invalid
clicks, we wanted to give you some additional background on how our
systems, processes, and teams work together to manage click fraud for
our advertisers, while also sharing what the overall landscape of
invalid click detection at Google looks like."

Some good graphs and clear explanations. As we have said before Google is generally better than their competitors at detection, but because of the volumes involved just looking at the 'invalid clicks' numbers still stacks up to around $1Billion per year.

She mentions that "every percentage point of invalid clicks we throw out represents over $100 million/year in potential revenue foregone" which kind of indicates precisely why Google is incentivised to not to do as good a job as they could...funny that.