I paid $140 for one on Ebay. It included the FM radio feature, 2 wireless headphones, and a game remote (one game DVD too). It did come from China and had a Sony DVD Player in it (They are still being sold now on Ebay). It's the drop down one from the car ceiling - 9inches. I then paid $110 to have someone from American Furniture & Electronics put it in ($80 without the FM feature). That was in June 2010. It still works PERFECTLY.
Amazon has them too (plenty under $150).
Walmart even has one for $99. My co-worker just purchased one from Walmart. She loves it. She just had to purchase wireless headphones which are $10 from Walmart too. Make sure you get one with wireless earphones (capabilities - ie: IR/FM Transmitters).
When I purchased the car the dealer quoted me $1100 for a 7" one.
Walmart 7" overhead one
http://www.walmart.com/ip/DP-Video-7-Widescreen-Overhead-Monitor-with-DVD-Player-and-IR-FM-Transmitters-DZ733D/16775720