I bought one of these units for £300, which was a great price I felt as they were going for nearly £600 at the time. The only other thing I bought was an aerial adapter from VW which cost £15
It all slots in without problems and even an idiot could fit it themselves. IF you do experience battery drain (which I don't), then you would need to buy a new CAN gateway, which again is easy to replace and costs about £70.
Anyone wanting to charge more than £50 for an install is taking the piss!