Click fraud isn't worth it unless you're operating an international ring of click fraud focused on search feeds, third-tier engines, and random ad networks. Those guys push an amazing amount of fraud, it's unbelievable. I was investigating $xx,xxx of click fraud a few years ago at the same time that I accidentally removed password protection from my personal US proxy server for 48 hours - my proxy was getting tens of thousands of hits per hours from Russian & Chinese IPs. The traffic was going to all sorts of random legitimate and illegitimate ad providers... including the campaign that I was investigating the fraud on.
In the end, I got a list of sites to block & enough evidence for the company I was working for to win a lawsuit re:nonpayment for the fraud traffic. Adsense click fraud isn't worth shit & unless you plan on getting in with Russian criminal gangs, don't even try.
That being said, if you do want to get in with the Russian criminal gangs... well, the feds will probably be knocking on your door sooner or later.