I would be quite surprised to see a $1800 RT C fare US-BKK, even on BR.
The cheapest C fares from the West Coast are $1,325+ (PR) and $1,588+ (BR) one-way. DL/KE/CI/OZ are around $2,000 one-way. You may be able to do better than that when using consolidators, but the best option seems to be a combination of BR E+ to BKK and a one-way/rt ex-BKK (from $850+ one-way/$1,500+ rt).