25 pesos an hour is a GOOD rate. Market rate is about 22-25 per hour. You´re lucky if you find better than that. Working 50 hours a week? Whoever said that is insane, it is just simply impossible. Working 30 hours a week is a LOT, if you consider 1. travel time 2. breaks 3. class prep. As the other poster noted, you don´t work 8 hours back to back like in normal jobs. You work one hour here, one hour there, another there...and you spend time travelling from here to there to there. A comfortable, full time Engish job would be 25 hours a week, but I recommend starting at no more than 20 until you get used to the insanity that is this profession.
Another thing that will lower your salary considerably : cancellations. The nature of teaching one on one is MANY cancellations - around 1 out of every 5 of my classses get cancelled every week, sometimes more. If the student cancels in advance, or your institute has no cancellation policy, you don´t get paid. So expect to earn at least 20% less than your original schedule should indicate.
So, 25 hours a week for 25 pesos an hour that´s about 2500 pesos a month, right? substract maybe 500 from that for cancellations, you can expect around 2000 pesos per month (maybe more, but not much) teaching 25 hours a week.
In sum, it´s not a well-paying job. But it´s not a particularly difficult job, and it´s not a profession. If it´s something to have fun doing for a year, go for it. But if you are like 90% of us, you will get tired of it - and be anxious to make some real money!!