Cheapest that I can find this at the mo
Designed for versatility and ease, the Xbox 360 Wireless Networking Adapter enhances your Xbox 360 experience in the digital home, seamlessly synchronizing with the Xbox 360 system. You can easily transfer videos and music to your Xbox 360 system from your Media Center PC, chat with your friends, play games via Xbox Live, and more-all without the clutter of wires.
* Dual band 5 GHz and 2.4 GHz Wireless A/B/G home network compatible
* Compact, high-performance design
* Powered by Xbox 360 to eliminate adapter clutter
* Streams photos and music from Microsoft® Windows® XP and recorded TV, movies, and videos from your Microsoft Windows XP Media Center - based PC