Well considering subsidies stopped and I got 450 for my two year old iPhone 6+. I put down 400$ of thay towards the 7+. Payments are about 20$ a month. Paying off the phone now would be about 250 and getting 600$ for on eBay will get me much more than trading in the phone.You won't necessarily get more if you try to sell the phone instead of using the IUP if you update at the 12-14 month mark. I compared the highest prices on the market to what Apple provided and they were about $100+ higher than market.
On the IUP, I don't believe condition of the phone really matters at trade in. If there is a major problem then an AppleCare+ claim is made at trade.
Remember I'm not considering that 400$ I put down as the phone was paid off.
How much are your payments with no money down on a 128gig or higher plus phone?
I forgot all the details that I figured out when I did this last year but sellling the phone for a good amount and putting. 4-500 down on a phone and repeating every year made more sense than paying 35-40 for a phone payment.