Well, seeing as the camera is retailing at 60usd, it probably only costs 20-30usd to manufacture. So that doesn't account for the entire hundred dollar difference. I suspect poor yields are likely a larger factor.
Thing is, if the Kinect becomes a big thing, and most multi-plats on the X1 take advantage of it, Sony can still keep that gameplay intact for the PS4 version, so long as its optional. Heck, if its gameplay using voice commands, they don't even need the Eye attachment, the Mic in the bundled headset should work in a pinch.
If, multi-plats don't take advantage, and the Kinect remains the sideshow it has been for the past several years, than Sony can continue to safely ignore it.
Either way, they're fine. Microsoft can take the risk now, and if it pays off Sony can bundle it later, after PS4 manufacturing costs start to decline.