We had a similar problem while i was in the army.
the temperary buildings we were in were shielded to prevent the wireless signal from getting out, which made it difficult to get signal in offices sometimes on the far side of the building.
Basically, we setup a regular DSL modem at the connection point, then ran a cable to a regular router in the middle of the building. Then, by setting up 2 relays (one on each end of the building with a LOS of the router), we were able to make sure that the entire building had wireless. Only problem we had was that it "leaked" through the shielding when doors or windows were open -_-'.
Oh, or you can just buy some uncut RJ45, 2 ethernet ends, and the clamp tool (this IS cheaper then buying a 70 foot pre-made RJ45) and measure the distance necessary. Once you have that distance, cut the RJ45. Strip the end and thread the colors. This is important:
Each side; white/green, green, white-orange, blue, white/blue, orange, white/brown, brown
Make sure it works (it can take a few trys till yer good at it).
Connect when you run the system, then disconnect and coil up when not running.
That, or buy a better wireless network card for your 360. The microsoft one isn't as good as some of the off-brand.