Well, regarding the domestic prices in the Phils. it´s quite simple...
The published US$ prices are valid in case you book your flight through a travel agency outside the Philippines.
So, the prices in PHP are (basically) only for sale in the Phils.
If you book your flight online via the airline´s respective website (if they offer that) you will only pay the PHP fare. Your credit card will be charged in PHP and your bank will convert the amount to your local currency depending on the day´s actual exchange rate.
So, it makes sense to book online or in case you want to fly on an airline without online booking possibility to book after arrival in the Phils.
Last year i made a short trip Manila-Clark-Manila outbound on SEAIR and back on Asian Spirit. I bought the resp. one-way tickets almost a day before departure and i paid on SEAIR ca. 15,-USD and on Asian Spirit ca. 20,-USD (all incl. tax)
As far as i know the following airlines offer online booking facility: Air Philippines, Cebu Pacific, Philippine Airlines.
Asian Spirit and SEAIR offer booking only with a booking request and after payment the tickets can be picked up at the departure airport on date of flight.
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