Found this article online.
After a long wait, T-Mobile will finally start selling the iPhone 5 this Friday. To sweeten the pot for people (and to encourage people joining up with Magenta), T-Mobile will be accepting old iPhone 4 and 4S phones for the price of a brand new iPhone 5. That is, if you trade in an iPhone 4 or 4S to T-Mobile, you won’t have to pay anything up front for a T-Mobile iPhone 5.
That’ll save you $100 immediately, since T-Mobile is only charging $100 down for a brand new iPhone 5 (compared to everyone else’s $200). Depending on your trade-in iPhone’s condition, you can also get another credit up to $120 to be used on monthly payments of the iPhone 5 ($20/month), your T-Mobile bill or purchase of accessories or another phone. Basically, T-Mobile wants to make it as easy as possible for you to switch over to its network. Which probably isn’t the best idea so late in the iPhone 5 cycle, but still. [T-Mobile]