Please be aware that Seoul has multiple airports and that these results are for all airports serving the city.
The best month to fly between Seoul and Manila is March and the most expensive month to fly is December. (Average prices, based on 1105 datapoints.)
The cheapest airline flying from Seoul to Manila is Jeju Air. They are 46% cheaper than Korean Air. (Average prices, based on 1112 datapoints.)
The best time to fly is at around 0. (Average prices, based on 1163 datapoints.)