The reasons I think, predict...and hope...this will happen with Z4Ms are based on precedent and are as follows:
Any car/model where this is likely to happen needs to fulfil certain criteria.
Firstly, it needs to be rare but not vanishingly so. Rare because it makes it desirable but it mustn't be too rare otherwise it's not worth hyping because there aren't enough units to make it worthwhile to dealers who want to make their money time and time over.
Examples from my own past experience that fit that criterion would be the E30 M3, as already mentioned above, but also things like the Mercedes SL Pagoda. Both are rare-ish but by no stretch of the imagination vanishingly so. IIRC E30 M3s were produced to the tune of over 17,000 units. Mercedes Pagodas were produced to the tune of nearly 49,000. Ok, many will have rotted away over time but they are still hardly "very rare".
http://www.bmwclassics.co.uk/gallery/index.php?spgmGal=Mercedes_Pagoda
So, the Z4M (Roadster or Coupe) certainly tick that box? They're rare (I don't know the exact figures but you guys will) but not vanishingly. But there's more.
Secondly, it needs to be something aesthetically eye-catching. That can be genuinely beautiful or even just quirky or standout. One assumes that most of us here like the E85/E86 styling? But, many people don't! It doesn't really matter. What is undeniable is that they have a distinctive, standout look. Most people don't buy 'classic cars' for their performance figures alone; they buy with their hearts which in-turn are lead by their eyes. My previous two examples also tick that box but I'll offer a further stark example from my archive: The BMW E9 Coupes.
I owned one of these cars for 13 years (1988 -2001). I bought it with some savings plus my first month's salary when I first qualified. At the time you virtually couldn't give them away and for my one owner, 70K mile, manual/carb version CS I paid the princely sum of £1500...mainly because I simply couldn't afford the £3000 that a full-blown CSL would have cost me. Look at them now. Again, a rare-ish car...but not vanishingly so...and with a stand-out styling. I don't have any decent pics on this computer but if anyone wants a blurred view (it was pre-digital camera days) it's the fifth thumbnail from the bottom in this link:
http://www.bmwclassics.co.uk/about/index.html
Thirdly, performance always helps. Today none of the above would be considered strong performance cars. I got rid of the E30 M3 because it was too flipping slow on the public road! But, in their own day they were all considered fast cars...and when you're a young schoolboy that's what you're interested in! Trouble is, when you're a young schoolboy you can't afford one. You have to wait until those teenage schoolboys have grown up, earned a few quid and can fulfil the dream that they had when they had their noses pressed up against the showroom windows. That's a difficult and imprecise length of time to predict but I would suggest possibly not less than 10-15 years?
There is a further (even to this day) persisting alluring dimension to performance and the 'Need for Speed'.
We still, illogically, hanker and desire the fastest iteration of any model. Hence, take the E46 M3 as an example. That is a car which, let's face it, is far faster than most of us could seriously drive or run out of performance in when driven at full-tilt, even on a race track let alone the public road where the fast majority spend their time?
But, somehow that's not quite enough performance. We think we
need the CSL! We need and want it simply because it's the fastest possible of the range. The fact that it's insanely fast, totally unusable for more than a few seconds on a public road doesn't really enter our serious decision making process. It's the fastest E46...so that's what we've got to have and can be led to believe that we're justified in paying a premium many times over the 'next best'. I know this because I did it myself!
I paid £30K for a CSL when, at the time, an equivalent normal-E46 M3 would have been sub-£10K and would still have been a car way faster than I "
needed. Once I had realised this, I then really struggled to sell it again for £28K. Again, the usual story played out.a A few months later the usual motoring journos and the usual dealers started casting their evil spell....and we know exactly what happened?
http://www.bmwclassics.co.uk/gallery/index.php?spgmGal=2003_CSL
So, so far all of the cars above we have:
1) Rarity...but not vanishingly rare
2) Stand-out styling
3) Appeal to schoolboys who've now grown-up a bit and have a few quid
4) Top-of-the-range performance....doesn't matter whether that's actually too slow or stupidly fast
Hmmm....what car does that remind you of?
ive never appreciated the styling & interior is "quirky" to say the least , , fortunately for me the values are irrelevant so no matter what ££ was on the screen i still wouldn't go near one
). Looking at it from that point of view I think the value in these cars at the moment is clear.
? 
