Since I went through this thing at least 3 times in the past 5 years of owning my truck I'll sum it up right here.
System itself - $600+. Anyone who knows anything about a computer, can probably build one, I know people with full towers in their trunks filled with parts from old computers, so who knows, this might be free for you depending on what you have in your closet.
Power supply $75+ - You can go cheap and use an inverter, but that is, the cheap, halfassed way out. Buy a real dc-dc automotive power supply integrated with your ignition system with real time battery monitoring. I have an Opus 320w which, unfortunately, is not made anymore, I think it cost me $225 back in the day.
Monitor - $200+ - You get what you pay for, and I'll leave it at that.
Misc stuff $75-150 - gps antenna, cables (usb extension cables, some 18gauge wire, your basic amp wiring, etc)
Any fabrication work - $0-??? - Most of the monitors come with stands and believe it or not, many people just mount the thing to their dash with velcro, screws, etc. People use cupholders, whatever. You want someone to build a fiberglass dash like mine with the monitor molded into it, plan on paying a decent amount if you want the job done right.
Knowledge and how to do this - A decent amount. This isn't "hey I know how to use facebook, I can put a computer in my car". The install is going to take as long as you want it too. But at least an entire day, and that's if you have some idea what you are doing. I have been doing building, assisting, and tweaking these systems for 6 years and I am still learning.
There is plenty of info out there on the web to do this yourself, but not many people are comfortable jumping into it, and end up taking the safe route up and plugging in their ipod =)