They are about £150 in Uk depending on model. You connect them to the back of your radio head unit in the CD changer input (need to take unit out to do this) as long as it has the 'MODE' button - and the normal fitment is to have the dice unit in the glovebox.
These kind of units are ideal for Ipods. They charge the Ipod, show the track on the radio head unit display and allow use of steering wheel controls.

That ^^ is why I prefer the much simpler/cheaper aux socket connection for my Ipod. Its a downside that the aux socket doesnt charge the Ipod or integrate with the display/steering wheel controls but I can live with that personally.