Interesting question. And I have some time to share a couple of thoughts, so here it goes
Mine was € 48.000 when new according to the insurer, in October 2006.
I bought it used at € 27.000 when it was exactly 4 years old and had done 38.000km.
So in the first 4 years € 5.250 depreciation on average. I am assuming here that a) the list price was never paid and b) the discount when new corrects for the part exchange value and the price I bought it for.
At the moment comparable cars are on the market for around € 13.000 and soon the car will 5 years older than when I bought it. So I'm looking at (€ 27.000 - € 13.000)/5 = € 2.800 annual depreciation. Actually that will be more since part exchange value will be around € 9.500. (boils down to € 3.500 annually)
Had I bought it new my average depreciation would have been around € 4.300 annually, which is remarkably similar to yours.
In the Netherlands, the value of the TT Mk2 seems to suffer a bit from three factors
1) lot's of imports of cheaper low spec TT's from the German market drive the price down
2) it's not a Mk1 (can you hear them cheer at the other side and say we told you so?)
3) there was never a huge demand for the Mk2 here
My estimate is that well maintained TT Mk2 will find a bottom value between € 3.500 and € 5.000. And with well maintained I mean: good brand tyres, no dings and dents or scratches, a fresh interior and technically fully up to date. Those that are neglected will continue to drop in value until they reach scrap metal value. By the time that supply dries up (I guess we're talking 2015 + 15 years, so 2030) the TT may start to increase in value again, depending what kind of world we live in by then and what the tax regime will be with regards to older cars.
Comparing to the Boxter is interesting. It's a Porsche after all. However it's not a desired Porsche. It's the 'you couldn't afford a 911' model just like the 914 was back in the days. The tables have turned for the 914, but that took a long long time. And perhaps only because a nice air cooled 911 is becoming very expensive. Will the R8 do the same to the TT? Become very desirable and unaffordable so people take a TT instead as it being the nearest thing? I don't think so. The current Boxter/Caymen duo will beat the current TT hands down. Porsche managed to turn them in proper desirable sports cars that seem to have a career of their own, whereas the current TT... sorry but yawn.
The Z3/Z4... I think they're not rare enough and they don't seem to have enough predecessors that are in huge demand and are increasing in value. I reckon the Z3/Z4 and the TT will remain pretty comparable cars.
I can't make my mind up where the SLK will go. I never looked at it much. Obviously I'm no yard stick for the market, but if most of the market don't look at the SLK much, then we all know where it will end: in the history books.
The Corvette, yeah the American way. Raw v8 power, chest hair, sleeveless T shirts, cowboy boots, tattoos, beer and barbecue. Attractive for some, for others not so. Plays in a league of its own. But none of the later models will ever come close to the original. Same will apply for its value.
Alfa Romeo 4C.... where will that end up? Way too early to tell. But at the current state of play Alfa Romeo is barely alive. That's never a good thing. But mind you, I do like the design and I loved the way my 156 handled and looked. Still even the special 156s don't seem to attract very much TLC. Anyhow it won't become the new Delta Integrale or Stratos, but then made by Alfa.
Having said all of the above I can hear the RS owners think... he's wrong. Yes I was looking at the 1.8/2.0/3.2 TT/TTS versions. I believe the TTRS will be the one that really holds its value and will be the first one to gain value in the future. In fact perhaps the TTRS should have been the only TT version in the Mk2 range. I only own a humble old 2.0 and would even have settled for a 1.8. So I'm glad Audi made those too. But the RS... that's the one.
While I was writing this post, my TT depreciated another 50 cents. Hope my post was worth it :lol: