What do you guys think is reasonable discount for 9 years of missing logs? Its been flying fine for the last 30 years, so if there is any hidden damage history during those 9 years of missing logs, I'm not too concerned with that, I'm more concerned that I get the right valuation and don't over pay if I decide to go with that airplane.
With it being the first 9 years of it's life, I honestly won't worry about it. Not that it's great that they're missing, but more than likely, most things have been replaced/taken care of on that airplane since then, the engine has likely been overhauled since then (and you can find the log entry for it), avionics might have been replaced, etc. You're unlikely to find anything in those missing 9 years that would affect the plane's value today, so they're not of great worth to you.
There's some variations on how missing logs affect the price of the plane. Some say 5-10% automatically. Some say it depends on what period of time the logs have been missing for, if it's greater than 10-15yrs and the plane has had major work since (engine OH, for instance), then it's negligible.
Just my $0.02...

I'm sure others will have better advice!