Which phone? Different phone models and different storage specification have different prices.
Here are the details of Apple iPhone Upgrade program.
iPhone Upgrade Program - Apple
Phone Model 6s 6s Plus
16GB $32.41/mo. $36.58/mo.
64GB $36.58/mo. $40.75/mo.
128GB $40.75/mo. $44.91/mo.
As to how much you will have to pay the carrier, which is indeed a separate and additional monthly cost, that will depend on 1) the carrier you select and 2) the monthly service plan you choose with them. You should contact your carrier of choice and see what their plans cost.