While Microsoft's first attempts at a motion-sensing peripheral were relatively primitive affairs, the very first prototype of Kinect was worth an absolute fortune.
A report on the New York Times says that the first working version of what's now known as Kinect cost Microsoft $30,000 to build. That's not the cost of the entire project. That's just the cost to build one camera. Expensive!
Things are now a lot cheaper, of course, with Microsoft selling Kinect for $149 and claiming that every unit sold is being sold at a profit. Most of that is of course down to the economy of scale, mass production and R&D, but let's not also forget that Kinect was originally intended to be a more powerful device than the one we'll be seeing next month.
With Kinect, Microsoft Aims for a Game Changer [New York Times]