Nintendo proved that expensive attachments can work. They did it with Wii Fit, which is highly successful.
It sells for € 85, but it includes a few really great games. That makes the price payed for a new feature okay, since the new feature costs € 35, taking into account that games usually sell for € 50.
What Microsoft is trying to do with Kinect is similar, create a new feature, and attach games that support the new feature. The proposed price of € 130 is a tad too high, especially considering there are no games bundled. They expect people to buy a feature, and buy the game that supports that feature for € 50 extra.
There's also a chance that it won't pick up, since all the casual gamers are using the Wii. Xbox gamers usually are online multiplayer fanatics, and probably not interested in jumping and arm waving by using a glorified EyeToy.