One thing you can do is go to the Used Cars tab above and appraise the cars you are interested in. After you plug in your zip code, options, color, etc. you'll see TMV numbers. Under them is a Certified Used Car price.
That's the number you should try to beat.
Certification is a bit of a joke I think. The manufacturers have a "100 point" inspection for the dealer, but you have to wonder what that really covers. Is checking the tire pressure one point or four? The inspection is probably much the same for any used car they put on the lot.
Even though the car is "certified" you really should have your own mechanic do a pre-purchase inspection. Lots of stuff can slip through the cracks and the dealer will simply rely on the extended warranty to keep you happy. They won't be concerned with your hassle for having to take the car in to get fixed.
Some people like having a manufacturer backed extended warranty (and you could probably buy one of those on any newer used car). About the only advantage to going the CPO route is that you may get loaner cars if you do have to take the car in for repair.