Are there good but affordable international schools in Manila?
There are a good amount of international schools in the metro. I can't attest to the price of each and every one, and I can imagine they get rather expensive. In any case, I found the definitive list of all of the schools. Perhaps you can do research based on which area you prefer and go from there. Here's the link: https://en.wikipedia.org/wiki/List_of_international_schools_in_Metro_ManilaMagbasa pa
There are the following alternatives that are significantly lesser expensive then ISM yet with great location in Metro Manila: Australian International School Beacon International School San Augustine School M. Gandhi International School Chinese International School Everest Academy Saint Pedro Poveda School European International SchoolMagbasa pa
Among all the schools that you mentioned, Poveda is the one I hate. It's a Catholic School technically but they're not pro-poor. Shame on them.
Technically, those schools are not cheap at all. It will cost you quarter of a million above. An ordinary or let's say a regular corporate manager can't even afford to enroll his kid in an international school. In fact, majority of those schools use a foreign currency for their tuition fees.