Well this will depend on what kind of plan you have? Have you converted to a Share Everything plan? What device are you using? A BB 9930 or a 9650? If you have the 9930, why use phone as a modem when you have a mobile hotspot(be sure to convert to a share everything plan). If you haven't and/or you have a Bold 9650, I believe tethering is an extra $20 dollars a month up to 5gb of data. $0.15 for every mb over I think.