Depends a lot on mileage and condition. At auction, it's not uncommon to get one with the usual wear and tear for the $8K range. If they have low mileage, that price can go up a couple of K. Over 100K miles, the price drops. You can find them on reseller lots in decent condition in the $15-$18K range, depending on condition.
Because of high-idle times, some seem to succumb to the dreaded DOD lifter failure, but the repair seems to work okay, and folks have a LOT of miles on the same engine after this is repaired. Our 85K '12 had the cam and couple of lifters replaced while in service at about 40K miles. It runs fine. I may some day do a cam update, and if I do that I'll remove the DOD, but until then.
Many G8/SS/Camaro/CTS parts swap into these cars, offering a lot of options to build one for you. You have to pick and choose, though, as some 'obvious' swap parts won't (the PPV has about 3" of additional wheelbase as opposed to the G8 and SS, most of it in the back seat area, for instance).