Over the years I've been in the sport (46 now) I've seen a fair number of gliders sold without log books, or with "reconstructed logbooks" etc. I also once saw a bank repossess a glider and the owner "ransom" the logs for $500 to the bank's new purchaser. A lot depends on the glider type (does it have a low maximum life limit?), desirability of the glider and sophistication of the purchaser. Probably a 10% value reduction is a good estimate. For me - with the glider you describe - a bigger issue would be potential water and vermin damage, brittle tubing and similar age/storage issues. I think the "Ds" were a little more saleable than the flap only version. You may also find this link helpful:
https://www.traficom.fi/sites/defaul...n_20151115.pdf
Good luck
ROY