XBOX 360- Wireless

Hello, It's really annoying me with a problem I've been occuring over and over again. I don't understand how to connect a wireless virgin broadband mobile internet source that connects by USB and only USB (i.e. can take in car and use on laptop) to connect my xbox 360 to xbox live. I have a lap top, and that is all in terms of computers, I bought a 1 year membership and I'd really like to use it >.< Any help would be thanked for.

P.S. Most websites i go to says "Buy a $70 modem to connect through wireless to your homebox, do I need to spend $70 for wireless modem -.-?