That depends heavily on what you have, what you do, and what your city and state taxes are. Find out this information:
a) What plan do you have? A voice plan can vary in cost from $29.99 to $139.99 (300 whenever minutes, 5000 whenever minutes with myFaves), and Sidekick costs $20.00 as an add-on. The Sidekick plan itself is $29.99.
b) How have you been using it? Have you downloaded any games? Ringtones? Are you making calls with the $29.99 Sidekick plan? (If so, those minutes are $0.20 each.)
c) Well, your taxes are decided by the government.

To get the definite answer, ask Customer Care. You can call or email them, and the info is here: T-Mobile.com

Thanks to justarep for providing this info.