I recently had my turbos replaced under warranty. My SA estimated $6000-$7000 if I had to pay for it myself. But apparently, they charge BMW around $4000 for the warranty work. Nice way to bend over your customers
I did talk to the mechanic who was working on my car and asked if I would have the wastegate rattle again, his reply was that BMW had redesigned the turbos and I shouldn't have the problem again.
If my turbos start to rattle again, and my car is off warranty, I'm going to go aftermarket and upgrade the blowers. It'll probably be cheaper...