I can't see it being that expensive. It shouldn't add much (if anything) in terms of weight, and should be a part of a basic requirement for a safe and comfortable work environment.
I there any other idea why this isn't a part of the basic equipment in the cockpit?
Glad that Beechcraft started something, I'd really like to know what came of it. I really couldn't care less about cabin noise though. Pax are only in the cabin a few hours a year, plus it's quieter back there anyway.