$150
Expensive yes but high-tech always costs a bit.
And if Sony made this first, it would probably cost at least $100 more.
To buy an HD-Wii, er I mean the PS3 Move, it would cost $50, assuming you already have a PS3. If you wish to play with your family it will cost another $90 to buy 3 more carrot-wands controllers. So that's a total of around $140 (although this is excluding the cost of batteries for each controller) in order to play 4-player Move, vs the Kinect which is will already support the family as is.
So yeah, either multiple cameras that keeps track of your body, face, and voice in order to control the game, or just 1 camera that follows a controller, that mimics a Wii-Remote with the extended Wii-Motion-Plus sensor.
Edit: Also, with Kinect, you can verbally tell your Xbox to play, stop, pause, and resume a movie with only your voice. There are also many other commands you can do, but I don't know them all yet.