From here:
Five Honest Tips For Starting A Car Collection That Won't Lose Money
Tavarish:
What about the red-headed stepchild, the 996? Do you think that it'll appreciate?
Zac: The desirable ones have already started to come up a bit, but they did get really cheap, so they probably only have one direction to go from there. They may start to appreciate a little bit, but I don't see it that much. If you take a 911 turbo, there was a long time where you could go pick up 996 turbos for $30 grand all over the place and now you don't see them as much, they're $35k - $40k, so there's been some appreciation.