While Microsoft HQ has yet to confirm an official rrp for Kinect, the company's own online store has no problems pricing the device at, yes, $US150.
What you're seeing here is Kinect's listing on the Microsoft Store, the company's very own internet shopfront. Now, when retailers like Walmart and Best Buy say Kinect is $US150, Microsoft can always say they're external parties, simply guessing, and until an official price is released, we're in the dark. But, uh, when it's your own store?
Less likely. I'd say between the retailers, the costing report and now this, it's safe to pencil in a launch price of $US150.