Regardless of the time of day you decide to fly, the ticket price will remain around the same. Book flights from Koh Samui to Kathmandu as you normally would.
The cheapest flight price from Koh Samui to Kathmandu is ฿31,013. On average you can expect to pay ฿31,013. The most popular route, (Ko Samui - Kathmandu Tribhuvan), can usually be booked for ฿30,255.