I tend to agree with that... however, I think this purchase is less about the overall drive / performance etc. and more about the whole aura of owning a 911; perhaps you could call it a bucket list car.
I also think because the 911 has changed so little (visually at least) over many, many years, that slapping a private plate on a 10 year old one, still has most of Jo public thinking its more expensive than it really is.
There is also the fact that once they've lost most of their cheddar, they seem to level off somewhat, and always command a lot more money than a rep mobile that has 150k on it, even when they are 10-15 years old.