In "Google's House of Cards" Nate Elliot states that the lowering quality of clicks from AdSense ads is erroding the ROI on AdWord accounts. He believes a second set of bids should be applied to AdSense ads which allow advertisers to bid on those ads at fair market value (less than the price of other ads).

While it may be a bit more work for the search engine marketers, Google already has this feature.

Set up your account and ucheck "the content sites in Google's network" box. This will disable content sites.

Set up a second account (with your lower bid prices) and enable the content sites.

Sure its a little more work, but why should Google harm itself and its content providers to provide a feature which is already there?