We have long known there is a massive difference in the way internet users respond to paid search ads vs contextual ads, and the pressure from agencies about this was hopefully what inspired Google to split their bidding systems late last year.

What cool's to see is this very well done study "An Investigation of Pay Per Click Search Engine Advertising: Modeling the PPC Paradigm to lower Cost per Action" from Alexandre Douzet from theLadders.com  As described by John Battelle it's a good introduction Pay Per Click advertising and many campaign optimisation principles, however for advanced advertisers there are also some useful graphs comparing the value models of both search methods.