Originally Posted by Diplomatico

People aren't suggesting that you fly from your home country to Chiang Mai roundtrip. They are suggesting you book a separate round trip flight from BKK to CNX. Those flights are very inexpensive (I doubt you'd pay $100 r/t) and take just an hour.

If you stay with the train option, I suspect that after the first leg you'll reconsider the flight option back to Bangkok.