1980 R100T Value?

So, my wife's cousin has decided he wants to sell his 1980 R100T and has asked me to assist with obtaining a value. Obviously I want to buy it but I want to make him a fair offer. Bike has 27000 on the odo. Tank and front fender has been resprayed.

I want to get it for as cheap as possible without causing a rift in the family!  :biggrin:

Can you guys give me an idea of what would be a fair offer?