100 pesos may seem reasonable, but maybe you should take a survey of the average income of memebers in your area and the parents of your students....it could just be to much for them or you could ask them when they are dropping out what there reasons are for quiting...they might surprise you and answer.

I will be moving in about 4 years to Mexico myself and would like to teach English there also. Are you teaching in a school, home, or office setting?