The rarest Corvette ever build and the most expensive, is the 1963 Chevrolet Corvette Grand Sport. These Corvettes were designed by the Chevrolet chief engineer Zora Arkus Duntov back in the late 1962, with one purpose only, winning the GT class at the 24 Hours of LeMans. They supposed to produce 125 units, but unfortunately GM canceled the project and ordered the cars to be destroyed.
But probably you will never gonna buy one of those, neither you will see it on the streets. What you can actually buy one day is this, the Superfomance Corvette Grand Sport, which is an official replica of the original model. Back in 2009 the original Grand Sport was sold on auction for over a 5 million dollars.
Source: The Hooniverse