I agree and disagree with this statement because as long as your site is built around that keyword and is optimized then you will get at least 40-42%, but the key to it is optimization of the site and manipulating google into showing only ads for that particular keyword at that cost otherwise it's like blogdum said "...you will not earn even 5% of that keyword..." . And even that will be because there are other ads showing that aren't bidded upon and thus less cpc bringing the other cpc down as well....personally I make a nice chunk with adsense every month on just HPK alone