Negotiating a CPO'd W211

In shopping for a W211 wagon, I'm finding the dealers' asking prices to be less than NADA, Kelly's Blue Book and Edmunds suggested retail. For those who have recently purchased used from a dealer, how much less should I offer? It's hard without some documentation suggesting the asking price is too high!

Also, I would like to add the +24 month CPO warranty. Is there wiggle room in the cost of that as well?

Thanks for any advice.