It costs around $700 for a complete carPC system if you already have an amp setup. The biggest pain is the screen if you have a standard double din opening that won't fit a 7" touch screen (such as on many Toyotas and Scions).
Breakdown in pricing:
Lilliput Eby701 7" touchscreen $170
CarPC (includes power supply, dual core atom, 1 gig of ram, case, 200 gig HD) $300
Visteon HD Radio $60
StreetDeck $99 (RoadRunner is a free alternative but requires the purchase of additional navigation software)
GlobalSat GPS $34
Total: $663
It's certainly not cost prohibitive. In terms