Just to be clear, the Carportal system is required to make the LCD screen work:
http://www.dealercostcaraudio.com/kos-v500.aspx
The cost for the screen and carportal is likely to be around 600, unless you go with crutchfield and pay a 200 premium.
Cheaper alternatives could be using the mount for an IPod touch, Garmin GPS with MP3 capability, or other media players that fit the dimensions of the opening.