I sort of doubt it. From my experience at Overture/Yahoo there were certainly multiple classes of words where advertisers were willing to pay more than $20 (car

I sort of doubt it. From my experience at Overture/Yahoo there were certainly multiple classes of words where advertisers were willing to pay more than $20 (cars, jewelry, mortgages (at the height of the bubble), lawyer services (particularly malpractice and mesothelioma, which was the highest priced keyword for many years)). OTOH, maybe AdSense is limited to $20? Contextual advertising is a lot less valuable than search advertising.