They have 2 choices for transfer case.
https://www.realoem.com/bmw/enUS/sho...diagId=27_0065
27108643149 recently reduced to $700 from $4-5k (likely as a good faith initiative)
or
27107649725 not reduced (yet?)
You would also need a new mounting bushing, oil, and you should put in new flex disk.
No it does not cost $6000 in labor.
Alternatively you can replace just the sheared output flange (that's the part that generally fails, for whatever unknown reason) for about $300 plus labor. It may be possible to do on car with driveshaft removed.