The most expensive car is the Koenigsegg Regera for 1,701,700 credits.
You can unlock this achievement in the test drive. This can be done right at the beginning of the game after the Street Race HQ has been unlocked.
Navigate to the catalog select the Regera, go to headquarters and start the test drive by pressing X.
For me I couldn´t get 2km in one sitting because there is a timer counting down after a couple of seconds. So I tried it again and achievement unlocked after the first 500 meters.
thx to ORO ERICIUS on the Xbox true achievements website.