http://newsroom.t-mobile.com/news/iphone-6s-offer.htm
"And, if you want to hang on to your iPhone after your 18 month JUMP! On Demand agreement is up, you can at T-Mobile for $125 LESS than the full retail price. That’s just $524 for a new iPhone 6s 16GB. In fact, T-Mobile is the only provider offering iPhone 6s and iPhone 6s Plus at a discounted price."
The quote above pretty much states that your Buyout will be the remaining balance minus $125.
Your end payment won't be the same as the regular JOD. It will be higher. Like I said... this promotion is a better deal for people who just keep jumping when their 18 months are up. Buyout isn't worth the trade in deal, unless you're like me and trading in a worthless junker...
I just called by the way and they are willing to let me trade in my HTC Touch Diamond from Sprint I've been sitting on for 6 years!
Essentially you could just as easily buy a $20 Pay as you go phone and trade it in for the $12 off deal.