Dont buy tickets on short notice in the peak travel season

If you want the best prices, look for tickets after the summer season, and buy tickets ahead of time. Depending on an airlines pricing/fare rules, you might find a cheaper round trip ticket than a one way. As long as you use the outward ticket, you don't have to use the return. Cant do the reverse though, they ussualy cancel the return when outward is not used.
Also inquire at local travel agents in the Philippines.
Also, depending on airlines, it may be cheaper to buy the international ticket and domestic ticket seperatly. You have more choices, but it does take addtional planning. Otherwise you would be limited to airlines with code sharing agreements. Although sometimes Orbitz will issue tickets for multiple airlines.