Great question — answering as the UphillTech team:
The prices you're seeing are very high due to a combination of factors: Being a holiday both locally and around the world, plus the best season in terms of weather for much of the country, New Year's is absolute peak season across Thailand and Samui is a very popular tourist destination. Samui is a small airport and Bangkok Airways has an effective monopoly on flights, so prices are always higher than most Thai domestic routes. The specific 18,000-baht itinerary you're seeing on Jan 2nd is not a direct flight, but a connection via Chiang Mai in the far north of the country, so you're paying for two separate long flights instead of a quick hop. If you're not happy with what Bangkok Airways...
Full answer by lambshaanxy: https://travel.stackexchange.com/a/177202 (CC BY-SA 4.0).
Need it done for your business? UphillTech.