Random thought vaguely on topic:
I have a theory that a car's long-term value is decided by a combination of 3 metrics
1. Beauty (subjective, but generally universal)
2. Rarity
3. Significance
If you put a value of 1-10 on each of those and add them up for a particular car, I think you can establish a general ROI on it's value.
30 points would mean a unicorn....think Mclaren F1 or Ferrari 250 GTO. The true archetypal legends.
26-29 points lands you in the realm of the F40, 959, 300SL and others. Just shy of top-tier legendary status, but still extremely valuable. (the 993 I posted earlier probably goes here, due to it's unique existence).
15-25 points is the realm of the traditionally valuable car
You could have a beautiful, significant car (such as the Citroen DS, Acura NSX) that is pretty valuable, but lacking in the rarity department.
You could have a rare & significant car (such as a Chrysler Airflow) that is pretty valuable, but lacking in the beauty department.
And you could have a beautiful and rare car (such as a Porsche 993 turbo, ) that is pretty valuable, but lacking in the significance department.
at 5-14 points you're looking at, at best, a car that is either rare, significant, or beautiful, but not really a combination of 2 or 3 of these categories. That or a middling score in all 3. These cars are kind of permanently only of modest value For example, a VW Beetle is significant, but not rare nor particularly beautiful, as is a first generation Ford Focus. A Jaguar XK8 is fairly beautiful, but they aren't significant nor rare. A Lincoln Blackwood is rare but it sure isn't beautiful nor significant. A 350z is slightly rare, fairly good looking, and somewhat significant. All of these are worth 'ok' money depending on condition, but not generally 'a lot' of money.
Below 5 points you're looking at the real mingers. Not rare. Not significant. Not beautiful. The Chevy Cobalts or Dodge Dakotas of the world. Fated to eternal valuelessness.
Occasionally there is a radical singularity of suck, a 0 on the scale. The second generation Hyundai Accent, for example. Utterly insigificant, available in every used car lot, and heinously unattractive.
Now the one thing that does change over time is rarity. So cars that are beautiful and significant and common today might be a long-term value gainer, as their numbers start to shrink (like my 986...right guys??)