The retrofit is working exactely the same as factory installed.
The most expencive things are the control module and the eject box. I bought them via Ebay for 130Euro.
Additionally you need the BT-Antenna, the microphone and the brackets for control module and eject box. Together 60Euro. I bougth these thing at my local dealer. You can get the part numbers here: http://realoem.com/bmw/
Programming the vehicle costs 114Euro.
Total: 304Euro ~ 235$