I will quote directly from T-Mobile press snippet:
Don't forget, you will also have to pay tax upfront as well, so plan to give T-Mobile anywhere from $40-$90 in tax depending on the models overall cost.T-Mobile customers will be able to place pre-orders for the iPhone 6 and iPhone 6 Plus from tomorrow. The first shipments will go out on September 19th.
These smartphones will be available on the Simple Choice plan from T-Mobile. The carrier doesn’t bind subscribers to a conventional two year contract however they will have to make equal monthly payments in order to pay off the devices’ total cost.
The iPhone 6 and iPhone 6 Plus 16GB models will be available for $0 down from Magenta with 24 equal monthly payments of $27.08 and $31.24 respectively. 64GB models of both smartphones will cost $99.99 up front with equal monthly payments of $27.08 and $31.25 respectively. The 128GB models will be available as well for $199.99 up front, monthly payments come down to $27.8 for iPhone 6 and $31.25 for the iPhone 6 Plus.