When I say expensive, I'm only speaking from an AdSense user. For example, I use AdSense on my blogs. I sometimes get a click that pays $1.50 to me. I know that that's around 1/4 of what Google charges the Adwords client. So that click actually costs the Adwords client $6.00. I can only guess that that $6.00 click converted into a sale or it would have paid much less.

Of course Google never reveals their reason or strategy for what they do. It's hard to predict.

http://www.soundclick.com/bands/default.cfm?bandID=400209
http://guitar.answers.com/guitar/playing-a-right-handed-guitar-left-handed-upside-down
http://lefthandedguitarplayer.blogspot.com/