You may want to check out a site like autotrader.com to find another sunfire/cavalier that is close to your mileage, condition and options to get a ballpark price. If you want to get the best price, you will have to replace the head gasket and make sure the car is mechanically sound. It would help if the car would pass a safety and/or an e-test.
Or, you could try to sell it "as-is". My mechanic friends like the sunfires and cavaliers because they're fairly reliable, easy to work on, and parts are cheap and easily available. Good luck!