The "certified" program consists of the dealer going thru an extensive checklist of items on the car and bringing them all up to spec. From what I understand, this normally takes the dealer anywhere from 5-10 hours depending on what needs to be replaced plus the price of any non-warranty parts. This cost is then rolled into the cost of the car which is then passed on via a higher price to the customer. It is very good program as long as the dealer follows the intent of the program and doesn't just check the boxes. The extended warranty is decent, but it comes nowhere near the manuf. warranty. It covers most of the really big ticket items however. I think it costs the dealer somewhere between 1000-1500 for the warranty, so that also needs to be rolled into the cost of the car. Thus the price difference you will often see between certified and no-certified because the dealer has more invested in the car.
If you go non-certified, there may be another warranty option available to you. Ford (volvo's owner)has something called "total care" that covers almost everything, but has a deductible everytime you use it. I think there are different levels ($50, $100, etc.) It costs about the same, but doesn't include the certification process. Thus, you may save some on the purchase price up front since the dealer doesn't have as much invested in the car. If the car is still under manuf. warranty, get the dealer to take care of as many of those as you can prior to buying the car or before it expires.