It totally depends on the school you are teaching in, I would assume. I haven't looked around at other schools, so I don't know what their salaries are like.
I teach at a private school in Costambar, and the salary ranges depending on your education. But with your qualifications I think you should be able to make some good $$. The teachers with degrees make about $16,000+ pesos a month. More if they have worked there for a number of years.I won't post my salary on here, if you want more info send me a private message.
I do know some schools in Puerto Plata who pay considerably less then where I work, and the hours are much longer.
I work from 8am-2pm at the school and have 2 or 3 45 minute breaks during the day (when they have their spanish lessons, since I teach only in english) which I use to do my marking, so I won't have as much to do at home.
However I also tutor after school at some of my students' houses, and an average rate I charge is $200pesos an hour + transportation (if you don't have your own or they don't drive you).