It definitely depends on a few things such as where the house is located, how many kids, at what time, etc
As sivan said, 30-40 pesos would be normal but I would not be surprised if you were offered less. Generally, if you are babysitting for expats they will pay you more in that range because you are english speaking (which just makes them more comfortable). You should also consider whether or not you will be fed and/or given taxi fare home if you babysit at night.