I am new to BMW's and to this forum--I reply, not as the world's greatest authority, rather by coincidence, I have a quote and web page for the media snap-in adapter and held it in me hot little hand--I await (allegedly November) a new 2012 X3 Xdrive 35i with all the tech stuff...the dealer parts department recommended the media unit as opposed to the music unit.
I show part # 84 21 2 218 390 on the quote at $263.64 MSRP and $237.28 over the counter with a 10% discount. I found the unit on the web page:
As always, be sure you paste the whole thing--you can also see the page by going to the web page shown in the original post -- enter the part # in search--goes right to it.
By way of introduction: name here is John. I am in southwest Oregon. I have just ordered a 2012 X3 XDrive 35i with all options except MSport and headlamp assist, as my wife wanted to have something left for her to do while driving. I just couldn't push the M Sport package--car looks really nice, though.