Couldn't it also be more people are just buying the 250gig bundle?
All the parents I know who are buying their kids a Kinect bundle this year even against my advice (some of them, not all of them, don't really need the 250gig bundle) are buying the 250gig bundle because of the implied value. You can either get 4GB or for $100 more get another 250GB!
Also, if your brother is having troubles finding a bundle, check places like Sears.