Just a Idea. I got these homeplugs on the go in my house as i was having problems with get signal strength around the house. They plug into your normal power sockets and use the electrical wiring instead of running a ethernet cable. One next to your router and the other next to the xbox and away you go. Probably still works out cheaper then a wifi adaptor.
http://www.solwise.co.uk/net-powerline-pl-85pe-mk2.htm