Kevin,
I'd guess the $100 is for a used whistle being removed from an engine about to be scrapped. Hopefully $400 buys you a new whistle. BTW, I believe the whistle is a copy of a Nathan.
There used to be a website called "Voice of the Iron Horse" about steam locomotive whistles. IIRC, he had a feature on procuring one of these $400 whistles. It had some noticable manufacturing defects, so the author decided to re-work it, which involved some brazing, machining , and polishing. He ended up with a very nice whistle for less than you'd pay for a genuince Nathan.