Going on from what El Bob said,
Forget Glasses or any ohter guide price as they only apply when a dealer wants to screw you on part-ex.
But it does depend on many factors and you don't give us too many clues.
If its a fine example and you can see from evidence the bike has been looked after and whether the main failures have happened or are yet to befall it
Then it could be worth the asking price though unless concourse I'd say its all its money.
If its been someone's 'project' or a roost for their chickens and holed up in a barn/shed for years and is nowt more than the basis of a resoration project then walk away if he won't let it go for half his asking.
I've been somwhat shocked to see some airheads, (ableit Paralevers) going recently for silly money and they've been very good examples too. Most recently a '95 R100GS-PD Classic with a few tasty extras up for £3250!!
Also what's your intended use? Noticing what else you have in the stable, is it to have something to 'play' at the dirt with without ditching the missus' 650?, taking in a bit of a RTW or just to have a bit of GS Nostalgia in the collection.
These things are a total culture shock but incredibly addictive, and with strain you emotionally and financially but they simply oooze character like no other GS can.
In the end if you find one you like you'll pay what's being asked in spite of all this. Happy hunting - good to hear we may have another real GS in the club.