That $15 was a promotion. You will have to pay the full monthly price. Probably between $30-35 a month.
This is the reason I'm not doing JOD this year. I got in on the sweetheart glitch where the 128gb 6s Plus went for the regular 16gb price so my price is locked in until my payments terms are finished. If I were to re-sign a new deal to get a new 7 Plus, I would be locked into the much higher price point. No thank you!