If you're shopping for a GS the Flea Market in here is a good starting place to watch prices. All the book values go out the window. The owners are selling them for what they think they can get. A couple of weeks of watching bike prices will give you an good idea of what you can expect to spend. Yea, you have the folks that think their bikes are made of gold but then you will see prices that are really low. Patience.
It will also give you a good idea of what type farkles, if any, you want on a bike.
1999 BMW 1100GS
2012 Kawasaki Concours