Looks like the lock assembly is different whether you have soft close or not, and for the front doors, the part alone is $560/door. So you'd be out about $2,250 in just direct parts, factor in a few small parts, and say five or six hours of labor, and you'll easily be over $3k, or about 15-20% of the value of the car.
http://realoem.com/bmw/showparts.do?...35&hg=41&fg=40