I think you are forgetting one thing.That's full retail cost. BMW's cost is probably closer to $2500. And out of warranty, you can find used 550 diffs for a couple hundred bucks.
people driving BMW's who pay the maintenance fee aren't the type who go to the junkyard.
and the reason the junkyard is full of BMW parts is because that 2nd and 3rd owner may not be able to afford to repair the vehicle..
