Lets say you have a site with adsense. Would you add lots of high priced keywords on your page(for a better page rank) and pay those high prices to get higher paying adsense ads. Is there a cost benifit or does it balance itself out?
You should be able to make more if you target expensive keywords, but obviously you will still need to test it and find out what works best for you.
It seems like adwords is a great launching pad for a site to get page rank, traffic, then other methods of getting traffic comes into play.