I think it depends on the trim level of the car. In some models, it's a $4k upgrade, I believe in others it's $5k. Talking to Dieter Burmester extensively about this and how they were able to do this at this price, in relation to their home audio gear, he said, "a lot of the cost involved is the massive power supplies, hand matched output transistors (in the case of the power amps) and to a lesser extent, the casework."
Still, I don't know how they managed to get it all done for this price. I've had the opportunity to drive the Naim/Bentley system (also excellent), the B&O system in the Aston Martin and Audi, as well as the Meridian system in the Range Rover, (all covered in various issues of TA) but for my money the Burmester is the one to beat. While not confirmed yet, there is a high probability that the Burmester sound system will make its way into the Boxster/Cayman range next year.