Advance 'S' fares to BKK are available for $5180 from the west coast (PDX, SEA, SFO, LAX). With taxes it comes in just below $5400. The M fare for ~$3k is a good option too as suggested above.

Buy 2 tickets -- a domestic round trip FAR to a west coast gateway and an international roundtrip to BKK from there.

That's buying directly from Delta.

You can do even better buying through a consolidator.