$399 would be an entirely reasonable and competitive price for a pair of ANC over-ear headphones.
I think $399 is a bit high, when Bose and Sony price their top-end sets at $349 MSRP - and are frequently discounted to $250-$300.
Also, ANC isn't easy to do - it took Bose over a decade to develop a good system before anyone else. Even with the benefit of being able to reverse engineer Bose, it took Sony several iterations over several years to match ANC performance.
I would pay $350 only if Apple's over-ear set meets or exceeds the Bose ANC quality.
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Hopefully they'll do a soundbar next.
An Apple soundbar with tvOS built in would be pretty interesting. I wouldn't consider it for my main tv, where I have a proper receiver and speakers, but I would consider it for a bedroom or home gym tv.