Just to clarify - if you're not buying the phone outright, you only have to pay the tax upfront. Then, your monthly fee is based on the data plan you have and the payment plan (for the phone) you've selected.
So - to use my plan as an example:
I bought my iPhone 6 Plus on the Next plan; I only paid about $70 upfront for NY state taxes. Monthly, I pay:
(1) $15 for the Mobile Share plan (because I have a >10GB plan; it's $25/month for under 10GB). AT&T charges $40, but then discounts $25 for me being in the Mobile Share plan. That $25 discount is where the increase in plans comes from if an existing plan is kept versus going with the Next program.
(2) I pay $42.50 for the phone. It's the 20-month plan, which means I can trade in the phone at 12 months for a new one, pay for the phone outright whenever I wish to keep it when I get a new one or stay with the current phone and don't pay the 42.50 after 20 months is up and keep the phone I've just paid for. Remember, this is, essentially, an interest-free loan as it's a flat rate (take the price of the phone and divide it by 20, in my case). Also, this is a no-contract plan in that I only have to fulfill my obligation to pay for the phone. If you brought a fully paid phone to this Next plan, you'd have no contract (and you wouldn't have this fee).
Note that AT&T has three different time periods - 20, 24 and 26. What then changes is the monthly installment payment and the time frame in which you can trade your phone in for a new one.
(3) Last, I pay for a data plan, which is based on how many gigs you want a month. I have a large (30GB) plan because I have three other people on my Next/Mobile Share plan.
So, the bill is a combo of the plan access fee ($25 or $15/month), the phone installment plan and the data access fee.
IMNSHO, it's a good deal. You'll just have to crunch the numbers to determine if it's good for you.
Marilyn