The previous poster has kind of touched on the reason for the higher cost of car CD stereos - you need mechanisms to counter the normal vibrations and shocks you get in a car whilst driving, so what you will find is most have an anti-shock mechanism backed up by a data buffering system - very similar to that you get on many portable CD players now.
If it wasn't for the buffering and anti-shock mechanisms, your CD would be barely listenable to in a car - it would just stop playing every few seconds due to the shocks or skip hidiously.