I flew R/T from Chicago to Bangkok more times than I can count - spent 15 consecutive Christmas holidays here - and never paid more than $1.200 for economy seating. I just looked on-line now (Cheap Flights.com) and found a roundtrip flight from Chicago to Bangkok , departing Oct 12 and returning Jan 12 for 35,000 baht (U.S. $1,100) ANA/United.
I'm not sure where you found that $6,500 flight you mentioned, but if it's not for first class seating look elsewhere.