This link might be of interest to you guys. $299 for the video module which works with or without the nav option.
BMW E90 1 Series - E90 3 Series - E60 5 Series - E63/E64 6 Series - E70 2007+ X5 - iDrive TV Nav Video Input Interface For iDrive. Connect a DVD Player, TV Tuner, Reverse Backup Video Camera, iPod Video, Aftermarket Navigation, Car PC Or Other AV Sou
They have a couple of different cameras although I found one on ebay which looks just like the oem camera on the X5 for only $79. They also have a similar camera as the original poster for $69.00!
170º Car Rear View CMD Camera Special for BMW X5 - eBay (item 280419879364 end time Dec-05-09 18:35:06 PST)
I emailed the seller and he claims this camera is a higher resolution than the oem. For $69 I guess it doesn't hurt to try it out...