Not much I'd say, they hold their price remarkably well, much better than a 370Z, mint condition ones fetch crazy prices. Can't remember the exact figure my son in law paid for his 1994 911 about 5 years ago but I think it was very close to what I paid for my Zed brand new.
Pete
Now your in the 964 and 993 territory (air cooled) where you will be lucky to get one for under £25K