$200.  Unlike, say, Verizon, T-Mo doesn't prorate it, so it's $200
whether you cancel with 7, 70, or 700 days left in the contract.

Not surprisingly, it can be found on their Terms & Conditions page:

4. Term; Cancellation of Service. If you select a rate plan with a fixed
term longer than 1 month, then this Agreement will continue for the full
number of months selected ("Term"). You may cancel Service for any
reason by providing us with notice (we may require up to 30 days), which
cancellation will take effect on or before the beginning of the next
billing cycle after the notice period, BUT IF YOU CANCEL SERVICE OR
BREACH THE AGREEMENT BEFORE YOUR TERM ENDS, YOU AGREE THAT THE RESULTING
HARM TO US IS IMPRACTICABLE OR EXTREMELY DIFFICULT TO MEASURE AND YOU
AGREE TO PAY US IN ADDITION TO AMOUNTS OWED, AS A REASONABLE ESTIMATE OF
OUR HARM, A $200.00 CANCELLATION FEE PER NUMBER (which may be deducted
from your deposit or any amounts prepaid by you, charged to your card or
billed to your account). Our cost of providing your Service and Phone is
not incurred evenly over the Term. Our monthly charges and other rates
are based on the assumption that you will remain a customer for the
whole Term. You and we agree that it is reasonable for your rates to
include the amount of the cancellation fee. We may suspend or terminate
your Service for any reason or no reason upon 3 days notice (unless a
longer period is required by law). If you breach the Agreement, we may
suspend or terminate your Service immediately without prior notice
(except to the extent prohibited by law) and do the same for any other
service you receive under any other agreement with us. You breach the
Agreement by: (a) failing to pay any sum when due; (b) failing to comply
with any provision in this Agreement or any other agreement between us;
(c) becoming the subject of any proceedings under the Bankruptcy Code;
(d) becoming insolvent; or (e) your financial institution dishonoring or
returning for insufficient funds your check or credit card. In the event
of cancellation, you are responsible for payment of all charges
(including any cancellation fee) due to us under the Agreement, which
charges will be immediately due and payable. If we reinstate Service to
you after discontinuing Service, you may be subject to a credit check
and agree to pay reactivation charges or deposits. After the Term
expires, you become a month-to-month customer but are still subject to
the Agreement, as modified.