A classic Ferrari will always fetch a premium at a high-end auction. It's essentially become a universal law that will continue to exist until the end of time. If you want an old Ferrari, you can expect to shell out serious amounts of money.
Some fetch more than others due to a variety of factors. Some of the rarest (and therefore most desirable and expensive) examples are the 275 GTB NART Spiders, of which only 10 were ever produced. There are, however, a few coupes that have seen their tops carefully removed... and Ferrari isn't happy about them.
Since only 10 examples of the NART Spider were ever produced, a few enterprising Ferrari coachbuilders and customers teamed up to craft droptops of their own. The resulting 275 GTB coupes became convertibles, and NART parts could be fitted to complete the look. These are referred to as "cut cars" and there's a dip in pricing when a well-heeled customer steps up to purchase such a machine.
An original NART Spider is heading to auction at an upcoming RM event during Pebble Beach, and the pre-auction estimate pegs the price at between $14 million and $17 million. Meanwhile, Russo and Steele will be offering up a 275 GTB of their own. This car should still cross the block to the tune of a few million bucks, but it won't fetch anywhere near what the original convertibles will bring in.
Additionally, according to The New York Times, Ferrari would prefer it if these "cut cars" never existed. There's a rumor that they want the high-end auction houses to stop offering them for sale. This will preserve the value of the original convertibles, and in fact may push it even higher.