How much does a lost aircraft logbook effect resale value?

I've been approached by an estate of a former club member who's glider is stored at my club. Its a 1978 PIK20-D. They have gone through his entire belongings and have not found any logbooks for the glider. Our club records indicate the glider has not flown since 2003. Its been at the club since at least 1998. We have gone through the plane and trailer and no logs. Its a certified aircraft and our local AI said there is a process to start a new log book after an inspection. The question I ask is how much value is lost with no history of the airframe? I'm trying to help the estate come up with a reasonable value.