Not including the audio stuff, I have about $1200 into it so far. It's not hard to do, its much like assembling a barebones home PC, where you buy all off the shelf cpu, memory, hard drive, power supply, and USB gadgets. Even the screen was an open frame lcd kit off ebay.
Add a metra dash kit to interface with the car, buy a copy of windows 7 and load, then get the free open source or paid "Front End" software, of which there are several to choose from, and you got a car pc that does everything a high end navigation head unit does but more and its expandable.
The hardest part was cutting the metra dash bezel, and re-gluing on the cut plastic parts like fingers to hold the lcd screen in place. You dont even have to do that if you buy a 7" screen, but I wanted to fit 8".
If you see it, get it, for tomorrow it may be gone!