Siddons is talking about UK prices, but his argument works the same for the American retail listings. The Kinect pack comes with support for two players (six non-active) and the Kinect Adventures game, with no other accessories needed. That's compared to PlayStation Move, which gives you the main controller, a camera, and Sports Champions for $100. But Siddons says the "Navigation controller" will cost $30 more, and expanding to full controls for two players actually makes things more expensive than Kinect. Even two Wii remotes and Nunchucks will run you $140, and that's without Motion Plus add-ons. When you look at it that way, you can see why Microsoft believes Kinect is priced competitively.
Of course, Siddons is obviously speaking from Microsoft's perspective -- you don't necessarily need the Navigation controller (you can replace it with a Dual Shock, which most people already have), or two full players' worth of controls anyway. Still, even if that $150 buys an experience similar to what's on other consoles, there's still a question of whether you'll want to write that big check in the first place.