Microsoft is feeding prices into the wild in order to make people think that Natal is going to be expensive. Then when they stand up during the E3 press conferrence they are going to deliver a price point which is much lower than what people think its going to be because of these rumours. That way its going to seem cheap to many people because they were preparing for a price which was much higher than what actually eventuated.
I am pretty sure that $150 is far too much to achieve their goals. Remember they want most Xbox 360 users to buy and upgrade so it cannot be too much more than the base price of software which is $60. Many of the earlier estimates used more expensive TOF cameras as the basis for the higher expected price whilst these have never eventuated.