It takes three things to quickly escalate the value of a car that is approaching 20 years old, where at 15 years old it will see usually see its lowest valuation dip, in all its lifetime (when absent of abuse, neglect, 'customizing', or deterioration.)

Nostalgia of former youth who couldn't afford it when new but can now, new youth (16 to 30) that find fascination/appreciation with a forgotten classic from just before they even cared about cars, that is priced low enough that they can afford (especially when it was a unique sports/muscle car with a design ahead of its time and saw low production numbers), and a car with enough aftermarket or used-part resources to make it worth it (and being easy to work on doesn't hurt either.)

Being 'forgotten' is the best way to get a deal on a classic car - before people start to 'remember' again.

At 15 I picked up 'an old Chevelle' that was same age as me. Like new, 37,000 miles, $500. Fast forward 41 years and 58,000 miles today... still have that car. To me, it's priceless.

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