You're not the first person to bring up the idea of trying to get a cheap phone from TMo. Always read the fine print brah...
4. * Cancellation and Returns. Service Cancellation: You can cancel a new line of Service without paying an Early Termination Fee if you cancel WITHIN 14 DAYS of activating a new line of Service (30 days in California; other states may differ - ask your sales representative) ("Cancellation Period"). You remain responsible for all Charges incurred through the date of cancellation. To cancel Service during the Cancellation Period, you may be required to go to the place where you activated Service and return any T-Mobile Device you acquired at the time of activation. You must return your T-Mobile Device in its package with all original contents, undamaged and in good working condition with no material alterations to the Device's hardware or software. If you do not return your T-Mobile Device or if you return your T-Mobile Device in a damaged, altered or destroyed condition, we may take one or more of the following actions: (a) prevent your T-Mobile Device from working on any network; (b) charge you the Early Termination Fee; (c) elect not to process your Service cancellation; or (d) charge you the suggested retail price or the cost to repair a T-Mobile Device, (which may be greater than the price you paid), plus any shipping and handling charges. Device Refunds and Restocking Fees: For T-Mobile Device and accessory returns and exchanges, see the applicable return policy, which is available at your place of purchase. Some T-Mobile Devices and accessories may not be refunded or exchanged, and you may be required to pay a restocking fee.