Fab work was free since I did it all myself (and the amazing case that was made for me by a friend), parts were probably around $1K, including the DVD burner and sub but minus the screen. You can go cheaper if you use regular components, but I did laptop components to save on size and also because they are sturdier.
If you check on MP3car.com, you can often find people selling almost complete set-ups and snag a pre-made that way. Otherwise, figure out what you want it to do, lots of music= big hard drive, DVDs or gaming= fast processor, fast processor+ big hard drive= big power supply, that sort of thing.
Also figure out where you would want to put it. Poseur did his on the seatback in the hatch, I did it in the dash, some people go for under the passenger seat or where the spare tire goes.