If you take care of you iPhone it will pay for the upgrade via trade in value. My wife and kids all got iPhone 6s by trading in their iPhone 5. No cost to them except a $30.00 activation fee.
You are paying for your upgrade with your contract. I will admit that used iAnything holds its value very well. I am tring to get an old iPod Touch for my daughter’s birthday, and even the 8GB/2G models are still in the $50+ range.
The only way to know the cost of the phone is to take the no-contract price. You can deduct the used price, too, but the difference is still substantial.