It is basically a plug and play install apart from the need to have it coded to your car, I am just in the process of upgrading my Mk2 RNS-E to have the OEM Bluetooth.
Mine was a little more than £150 but not much and I was really lucky as I came across a brand new latest version BT module for a really low price as well as the microphone and antenna.
New OEM 8P1862335A BT Module - £100.00
Plug n Play Loom - £40.00 (With SDS Conversion as my RNS-E has been updated to use the SDS)
New OEM Microphone 8L1862373A - £18.00
New OEM BT Antenna 8P0035503M - £13.72
Just need the time to install it and code it....
I will also have a Parrot MKi9200 for sale once the above has been fitted, this is just Plug and Play as it comes with the Quadblock to plug directly into the rear of the RNS-E (It may work with others) and having to run a live from the fusebox.
It displays in the DIS that you are using the phone but nothing on the RNS-E, it automatically mutes the radio or pauses the media you are listening to. They do share the same Parrot system apparently so I'm hoping that the OEM voice recognition on the BT is half as good as it is on the Parrot then I'll be happy as that works flawlessly.