I sold one of my brother's to a friend for $750. It was a patrolman's carbine and it had the removable carry handle.
If it is just the A2 (no removeable carry handle), I have seen them new for $650 in the Gander Mountain circular that comes around.
This is what Bud's is selling them for: (Fixed carry handle carbine upper)
85% of new is usually a fair price in my experience.
I wouldn't go just real high, unless it is something you really want.
You can get a Colt 6920 at Wal-mart for $1,099 in my neck of the woods.