That rating doesn't make any sense. It is purely based on how many mygranturismo users actually own it in their "online" garage.
Some of the cheapest premiums are "rare", mainly because nobody wants them.
To the OP,
no car is rare anymore. The game exists for over 2 years and all cars are out in the open. Of course, one is easier to get then others.
But there are a few ways nowadays to get any car within a matter of a few minutes.
Cheers