We had to upgrade our 6 and 6+ to 7 and 7+ with a $650 credit for trading each in...admittedly that commits us to 2 more years with ATT to get the monthly bill credits to pay the phones off. If next year's iPhone 8 model is really revolutionary, we can use ATT Next to upgrade again....we should still be able to keep getting monthly bill credits for the first year with iPhone 8.
The iPhone 7 will cost ~$4/month (upgrade to 128gig), and the iPhone 7+ about $9/month for 24 months.
It seemed like too good a deal to pass up since I have no plan to discontinue our ATT service and our trade in values would continue to plummet if we held on for another year.