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.
What value is a phone with no service? Just use an iPod of iPad mini.