That generation of Grand Prix were extremely reliable and it was a shame to see the entire brand go away.
My wife and I got rid of our 2004 Grand Prix GT1 (non-SC base model...no options) about a year ago. Even though it had 167K miles on it, we were able to trade it with a contractor in exchange for the labor to put a metal roof and new fascia boards on our house. So if you were to translate that into currency, I'd say we were able to trade it for about $4-6K worth of labor. So depending on the mileage and condition, I'd personally say that $4600 isn't too bad of a deal for a car that is two years newer, with more features, and fewer miles.
Also, my friend was able sell his 2004 Grand Prix GT2 (non-SC 3.8L with: leather, moonroof, Monsoon stereo system, HUD, and heated seats) with ~200K miles in mildly rusty condition on the side of the road for $5300 (with a set of snow tires mounted on steel rims) about 2 years ago. So again, $4600 doesn't sound too bad to me.