My whole carPC cost me about £450-500 all in. I did a lot of the work, I already had a spare copy of XP, and a spare laptop hdd, so saved a few quid there.
Cheaper today i recon - and you can get an indash motorised Touchscreen rather than a std 7" tft which i moulded into my dash.
Think about it this way though - get a cheap laptop and remove the screen. Flog screen on ebay for circa £100+, and use the base of the laptop as the carPC. Then all you'd need is peripherals - usb gps receiver, screen, wireless keyboard/mouse, psu...
I recon I could pick up a laptop for about £200 ish, recoup a bit from the screen sale.. In fact, i was working on this recently:
Dell C400 laptop on ebay (1.4Ghz, 512RAM, 40G HDD) - £200
Sell screen from laptop - £80
In dash touchscreen - £180
Other peripherals - £100
hmmm... maybe i'll make another one some day