I don't know if you booked your tickets yet, but I checked KE and flights from WAS (IAD) to BKK are the followings:
Outbound March (25th-31st) and Return (April 3rd-9th) are from $1019.44-$1099.44)---These are for "Super Saver" no stopover.
If you want to do a stopover in Seoul you'll have to pay ~$350 more (~$1366 total) for the "Saver".
Hope this helps.