Please be aware that Seoul and Bangkok has multiple airports and that these results are for all airports serving the city.
The best day to arrive at Seoul is Monday and it would be best to avoid Friday, as prices are on average 104.42% higher than on Monday. (Average prices, based on 246 datapoints.)
The cheapest airline flying from Seoul to Bangkok is Jeju Air. They are 67% cheaper than Korean Air. (Average prices, based on 208 datapoints.)
