Adding a line on my T-Mobile plan?

I have a phone for T-Mobile I pay 110 a month for my phone. I want to add my mom to my plan, also with unlimited talk text and web. Anyone that works there know how much my payments will end up being? I want to get her the iPhone 5s they're free atm.

The 5S is not free at t-mobile? It is Zero down then $22.91 a month for the phone and $50 for the phone line, $72.91 total. T-mobile has no contracts any more.