At our shop, the most common reason for drive unit rebuilds these days is actually due to the coolant seals failing, and not nearly so much milling noise (in later motors, they switched over to ceramic bearings which mostly fixes the milling issue, but they also changed the coolant seal to an inferior design that does not last). A full rebuild is $4,000, assuming the drive unit is in "functional" condition and can be rebuilt.
If we were to do a full overhaul at the shop where I work, we would:
-Replace rotor bearings with upgraded hybrid ceramic bearings
-Replace primary drive gear bearings
-Replace rotor coolant seal with an upgraded triple lip PTFE seal
-Inspect all parts of the drive unit for excess wear (splines, gears, etc.)