How much you pay for excess baggage depends on many factors. For example, which airline you fly with, where you are flying to, whether you pre-pay for your extra weight in advance or pay it at the time you check-in.
With Cebu Pacific, if you pay for your extra baggage in advance the charges for international flights are:
15kgs extra - P400
20kgs extra - P580
30kgs extra - P950
45kgs extra - P1,700
Those rates seem quite reasonable to me and much less than they were many years ago. Before the days of the low-cost airlines, the legacy carriers used to charge extortionate rates for excess baggage. It was charged at the rate of 1% of the one-way First Class fare for every kilo of excess, so you could end up paying almost as much for your extra bags as you did for the flight itself. We get a much better deal these days.
By the way, Filipino nationals pay exactly the same rates as anyone else.