This 1971 Plymouth Cuda convertible is expected to sell for about $150,000 at this week's Barrett-Jackson auction. A car like this with a Hemi engine would be worth over $2 million.

NEW YORK (CNN/Money) - When people start paying more than $100,000 for 30- to 40-year-old Plymouths, Pontiacs and Chevrolets, some classic car collectors start to worry.

These days, in fact, a bolted-together clone of a 30-year-old Plymouth can go for six figures -- the real thing having gotten far too expensive. That's because the hottest trend among collectors is classic muscle cars.

"A lot of these cars that we're talking about are really, really great cars, but I think there is a frenzy in the air," said Mark Hyman who operates Hyman Ltd. Classic Cars, a collector car brokerage in St. Louis.

Craig Jackson, president of Barrett-Jackson, a classic car auction company, counters that muscle car values have risen steadily, even right through economic downturns. There are simply more people today who remember the muscle cars they lusted after as kids and now have the cash to satisfy that craving.

Barrett-Jackson's largest annual auction opens today in Scottsdale, Ariz. Several other auction companies also hold auctions at this time in and around Scottsdale. The Barrett-Jackson auction is known, in particular, for high-value classic muscle cars.

"Classic muscle cars" generally means two-door American cars, built in the decade beginning in 1964, with back seats and beefy engines. The Pontiac LeMans Tempest with the GTO high-performance option package is widely considered the first true muscle car.

But muscle car collecting has a twist. Many muscle car collectors don't care all that much about authenticity. Not that they don't want to know exactly what they're buying, but more are happy with a faithful copy or a car with some "improvements" over the original.

While that opens up the market considerably, it also makes predicting future resale values difficult, according to some experts.

Cars, it is important to remember, are collections of parts put together on an assembly line. Big car companies, like General Motors, Ford and Chrysler, use various parts, including body panels and engines, in assorted models sold under different brand names.

If a certain combination of parts -- say a 1971 Plymouth Barracuda convertible body with a Hemi engine -- wasn't originally made in sufficient numbers to satisfy modern collectors, those parts can still be assembled today.

Cars created this way are sold openly as "clones." They aren't worth nearly as much as originals, of course. For example, a very well-made clone of a 1971 Hemi Cuda convertible -- a higher-performance version of the Barracuda -- could sell for as much as $180,000, according to McKeel Hagerty, president of Hagerty Insurance, a company that specializes in insuring collectible cars.

While that might seem like a lot, authentic 1971 Hemi Cuda convertibles are, today, the most valuable of all muscle cars. Very few exist -- only 11 were produced, at most -- and the few that survive are worth over $2 million each. (When CNN/Money last wrote about this car in Sept., 2003, it's value was just approaching $1 million. Click here for that story.)

Still, the high prices commanded by modern remakes of classic muscle cars are a matter of concern to some enthusiasts.

Rob Meyers, whose company R.M Auctions is also holding a classic car auction in Scottsdale this week, called the six-figure value of Hemi Cuda clones a sign of "a dangerous market."

It is all but impossible to assign meaningful values to anything other than authentic, factory-original muscle cars, Meyers said. Modern recreations or extreme modifications of muscle cars are difficult to value, he said, since more can be made at any time.

Since so few originals exist, it would be virtually impossible to pass off a cloned '71 Hemi Cuda convertible as real. While prices for clones may seem high to some, at least buyers know exactly what they're getting.

With more common muscle cars, clone versions are sometimes sold as authentic.

"There's a very large percentage of cars being bought and sold today as originals that, in fact, are not," said Hyman.

While the problem of counterfeiting certainly does exist, it isn't so common that it should scare away potential hobbyists, said Jackson.

Barrett-Jackson authenticates all the cars sold at its auctions, Jackson said, and fakes have been caught. Often, the owner himself had been duped, he said, and didn't know it until a Barrett-Jackson representative turned away the car.

There are several ways to detect fakery, said Jackson, as well as ways to quickly check that a car has already been reliably authenticated by a third party. Care does need to be taken, he said, as with any big-money purchase.

Increasingly popular with muscle car collectors are "resto-mods." As the name implies, resto-mods are restored but they are also modified or modernized. From the outside, a good resto-mod looks like a faithfully restored muscle car.

Inside, though, it might have better seats, three-point safety belts not available on the original, disc brakes where the original had slower-stopping drum brakes or other modern parts that make the car more enjoyable to drive and safer than it would have been with "correct" parts.

In most types of collecting, any change from the original detracts value and it is that way with many muscle car collectors.

"Some guys want matching numbers," Jackson said, referring to identification codes used to track cars and parts, "all the way down to 1970 air in the tires."

For some collectors, though, modifications in the interest of comfort, safety or performance can add to a car's value.

"There is a greater tolerance for modified muscle cars than there is for, say, modified '50s cars," said Hagerty.

That's because many muscle car collectors like to drive what they collect. For example, Jackson even drives his authentic '71 Hemi Cuda convertible, he said. The original engine stays in the garage, though, mounted on a display stand.

How much variation is acceptable from the original is a matter of taste. Before making any purchase of a collectible car, it is important that the buyer understand exactly what's being purchased, experts agreed.

That means asking lots of questions, checking all paperwork carefully and, all the experts CNN/Money spoke with agree, enlisting the help of a knowledgeable appraiser. Also, they said, while a well-bought classic car is unlikely to decline in value, buyers shouldn't look at any classic car as purely an investment.

"It's primarily in the love of the car," said Brett Torino, a Las Vegas-area real estate developer with an extensive collection of classic muscle cars, including some extremely rare cars. "You don't get into it for the money."