A:Answer Greetings Sam,
The 6S Plus cost would vary based on how long you want to keep the phone for before upgrading to a newer one; The shorter the time, the higher the monthly price.
For AT&T, there's Next 12 (20 Payments, Eligible for Upgrade after 12) for $37.50; Next 18 (24 Payments, Eligible for Upgrade after 18) for $31.25; and Next 24 (30 Payments, Eligible for Upgrade after 24) for $25. Note these rates are for the hardware only and do not include service.
For Sprint, they roll the device and plan (iPhone Forever with Unlimited Data) together which for that size of device would run around $85/month, but anytime you don’t have the latest iPhone, you are eligible to upgrade. You would bring in your iPhone and upgrade on the spot and away you go.
Welcome and hope this helps!