GDF wrote:

Cost of adding an apple phone to an account

 

is there any additional cost and if yes, how much, to add an Apple Watch to the account (I am not asking about the cost of the watch but the cost of the service)?

If you are talking about a phone, as your title mentions, the cost depends on the plan you choose.

If you are talking about an Apple Watch, which is a wearable, the cost is $10/month with autopay.

https://www.t-mobile.com/cell-phone-plans?showTables=true