The wages seems quite low to me, it has been awhile since I was in the teaching scene in Mexico, but that works out 3600 pesos a month. One poster over on ESL Cafe is paying her nanny 2400 pesos a month in rural Oaxaca, so think about how much you are earning.
20 hours is a lot of teaching per week, anymore than that and your teaching will start to suffer. I think about the most hours a person could handle is about 25 hours a week before the workload and stress begins to show, I know because I have pushed 40 teaching hours a week in the past.
I am not saying to avoid the place, just that the wages seem way on the low side, but if you take a job with them, consider that you are giving them 30 pesos of charity for every hour you teach for them. If they are typical language school owners with a nice house, new car, etc, you will quickly decide if they are worthy of the charity or not.