cost to replace alternator is $1200 with the mechanic

i have found bosch alternator for this vehicle for 400 at autozone/oreilly

I have worked on my vehicles (not BMW's) all my life, so im very experienced at changing parts . In looking it up on how to change the part, it seems rather simple, remove fan cowl, remove alt. bolts, etc... just like most other alternators.

trying to decide if that is something i should do, guess im just a little intimidated by the BMW NAME. This is my first one.