From what I can gather at
T-Mobile's Apple Pre-Order FAQ, the promo will work as follows.
For iPhone 7 or 7 Plus or 8, you get $300.
For iPhone 6s or 6s Plus, you get $200.
For older iPhone, you get $100.
When you trade in the device at the store, mail in, or Costco, you will get T-Mobile's trade in value for that phone as
indicated here on your account as a statement credit.
The remainder of the balance will be posted as monthly statement credit for 24 months (if trading at the store or mailing in) or in case of Costco, prepaid MasterCard within 8 weeks.
For instance, my 128 GB iPhone 7 Plus has a trade in value of $247. If I trade in at Costco, I will get $247 on the billing statement and $53 will be mailed to me as prepaid MasterCard.
I think I may just go with Apple's Give Back recycling program, which offers $285 Apple gift card for my phone. For $15 more, I can buy an unlocked phone directly from Apple and avoid the hassle altogether.