I had this same problem I didn't want to spend $100 on the Microsoft one so I use this usb adapter.
Amazon.com: Linksys USB100TX EtherFast 10/100 USB Network Adapter: Electronics
Basically what I do is hook up the Ethernet to the xbox then usb to my pc, then I just bridge the network connections and run off of wireless from my computer works great. But the only USB device that works for xbox is there own kind of sucks but there are many alternatives.