There is no figure to answer your question.
Having lived on lot less than 700 and i can tell you it is possible BUT everyone is different. Would i like to do it again? NO!

Expenses in the Philippines vary from place to place. Rent in one province can be double that of another. Cheap rent may sound great but it will be nothing like what you are used too. It may seem exciting at first until the little faults in your rented house start to annoy you.

DO NOT underestimate the cost of living here as it is not as cheap as people believe it to be. Always have a back up plan for price increases as inflation and the strong pesos can play havoc with peoples budgets. I didn't but i was young and maybe foolish at the time!

Living a decent lifestyle here on a small budget is fading rapidly.

Having said all that the vast amount of retired foreigners leave the Philippines because they can not adjust to the culture and not for financial reasons.