The most budget-friendly way to fly from Koh Samui to Udon Thani is to grab a ticket during one of the major seat sales which different airlines run quite frequently. To do that, you must plan your journey well in advance.
Besides the route and the airline, the price of your ticket will depend on the class of the seat, as well as on the season you fly, the day of the week, the time of the day, the extras included in your rate – meals, checked-in luggage, etc.
The lowest standard fare for this route is USD 161.33 offered by Bangkok Air (บางกอกแอร์เวย์ส), Thai Vietjet (ไทยเวียดเจ็ทแอร์). If you are ready to shell out USD 280.70, you can fly with Bangkok Air (บางกอกแอร์เวย์ส), Thai Smile (ไทยสมายล์).