After spending 3 weeks searching the country for a 2009 G8 GT with the features we wanted, I can tell you that these cars are commanding a premium price. You can find private sellers with the occasional steal of a deal (found a 09 GT w/ sunroof and red/black leather for $16,500, but didn't have other features we wanted)
Dealers are asking $20-22K for 09 GT's loaded out with sunroof, leather, sports, and premium. They are getting 18-19 for them, all day long. If the car is clean, it doesn't sit on the lot more than 7-10 days.
We found one in Wichita that needed $2,000 worth of paint repairs, had worn interior, and a motor that seemed a little weak ... and the best price I could get them to was $17,900.
We paid $18,700 for a white one, loaded out, from a dealership in Indiana. Even it wasn't perfect (minor paint blemish here and there). But body was straight, motor was strong, interior was as fresh as brand new, 71K miles, original window sticker still in the glove box, exceptionally well maintained.
We might have gotten them down to $18,500, but they weren't going to make it easy to get that extra $200 out of them, LOL, so it wasn't worth the fight. They knew what they had. I knew what that had. We both knew the other knew that we all knew what they had.
Also, FWIW, our 2009 Sport Red GT, loaded out, 84K miles ... State Farm valued it at 19,300 when they totaled it out.