There are a few you can buy cheaper. It just depends if you want the compass etc or the homelink as well. I saw this just now thought i'd share.
not my auction, would make a good retrofit for just mirror/camera for pretty cheap, includes pigtail/pin out:
Autodim OnStar Rearview Mirror w/3.5" LCD BACKUP CAMERA DISPLAY | eBay
too bad the quality of this is pretty crappy if not would be a good idea:
http://www.ebay.com/itm/New-Starvisi...item2a18d5d8a0