my 15 year old charges $20 a day (usually only for a few hours) to babysit, I wouldn't think she would charge any different for being a mother's helper... BUT she also knows that she is being paid to care for and interact with said child. If I saw my sitter locked into her cell phone and not interacting with or watching my son, I think I would fire her on the spot. An occasional text message is one thing (I do that at work myself), but I also find time to do my job around texting (and Mamasource LOL)
So, to answer your question, yes, you are over paying for a mother's helper. I pay for experience.
I would let the girl you have now know that you are paying her to interact with your son, and if that is interfering with her cell phone time than she can and will be replaced. at $14 and hour I would expect to hear her singing and playing and reading books to him the whole time she was there. I don't pay my 15 year old (except the evenings I work at my second job and my husband isn't home) for sitting with my 3 year old, and I can still hear her playing and reading to him while I'm cooking dinner or doing laundry.
Good luck!