Many cars of a certain age will appreciate in price at some point. More so when the numbers of actual cars available is low. Couple that with opportunity to bring into great condition, good history and importantly interest from potential owners and the price rises.
Fiat Coupe has been mentioned and is a prime example. Yes there are quite a few dogs about, but overall numbers are very low nowadays. They are a quirky design, unavailable at the point of original sale in countries like the US (can be imported there now as the early ones are older than 25 years and for some 40/50/60, still a desirable car. They were also produced in different engine types and trim levels, which makes certain models even rarer. The very best examples are now commanding £25k......
Quite a few cars bubbling under the radar......