What makes a car a classic? A unique cultural formula of factors, according to vehicle valuation specialists at market research group Black Book — and if they all come together the right way, you could be making big bucks selling vehicles that were once commonplace. There’s just one catch. Well, two. It would be better if you bought it in, say, 1971. And it really helps if the car in question is still in mint condition.

Some cars once sold for the manufacturer’s relatively affordable suggested retail prices have years later been marked up thousands, sometimes tens of thousands of percent. The 1971 Plymouth Barracuda, for example, was once purchased for an asking price of $4,296 and is now worth $2.5 million — a 58,000% increase in value over time.

Once purchased for just $7,500, the 1967 Shelby Cobra has increased 17,000% to $1.3 million. Other models are also raking in cash in auctions around the country, the list shows, including the Cadillac Eldorado, Chevy Corvette, Pontiac Bonneville, Ford Thunderbird, and Ford Mustang GT.

Eric Lawrence, director of specialty products at Black Book said, the percentage increase is influenced by the three Ps: popularity, pop culture and production volume. “Many people think today’s uber-expensive vehicles always came with a hefty price tag, but these vehicles show that’s clearly not the case,” he said.

Every year since 2013 has brought record prices for classic cars—but the market is stabilizing. The prices paid for cars increased 25% in 2014 and rose by nearly 18% in both 2015 and 2016, according to the HAGI Top Index, a measure of prices paid for cars that cost more than £100,000 ($122,670). Most of the cars were sold in auctions around the world.

But there are headwinds to the amount these cars fetch at auction. Millions of baby boomers, the natural buyers for many of these nostalgic throwbacks, are now bracing for retirement and face an uncertain health insurance landscape as the current administration revamps the existing health care law.

There may be more offerings for wealthy Generation X buyers who are now in their late 30s and 40s, however. The kinds of cars being sold are also changing: as a new generation of buyers gets the spending power to collect their own cars, exotics from the 1980s have started to gain traction. So, hold on to your Prius, and see what it’s worth in 40 years.