If you've got a GSM phone with the right radio bands, I suggest taking a look at the $30 plans here: http://prepaid-phones.t-mobile.com/monthly-4g-plans
For CDMA I believe Sprint and Verizon offer prepaid plans without contracts at around $50 for talk/data.
The key is that IMO, a prepaid plan makes the terms of the arrangement clear, and if the carrier decides to try and overstep their boundaries, you can cut them off. They don't have the ability to take you to collections or withdraw from your account or increase your bill behind your back.
Of course, if T-Mobile tries to screw with you as a prepaid customer, they've got some leverage since your only alternative is AT&T. Not so great. Still, better than nothing...