I bought mine from Bimmernav, at their retail price. For me, it was worth it. I wanted full integration with the onboard computer, dash and steering wheel controls, voice commands, built-in speakers and microphone, and such. An aftermarket bluetooth gadget might give the ability to have a handsfree conversation, but there are a lot of options built in to the BMW computer those kinds of kits just can't touch.
Having one of those cheap kits is kind of like taking a sack lunch to a catered banquet.
Dave