I thought you wanted CX? So why are you flying on NW and TG? And why return from BKK to LAX?

TG ex-BKK is the most expensive of all airlines. CX in Biz is around US$2040 ex tax BKK-HKG-SYD rtn no stop in HKG permitted.

You'd still need the oneway from LAX - have you costed that in?

From BKK to LAX, it will depend on what airline you want.. OZ is the cheapest and oneway is around US$800. Others like CX, SQ, JL etc are more than US$1200.
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So if pieced together , you'd get in Business Class:
LAX-BKK cheapest oneway on AIRCANADA - US$1425 b4 tax
BKK-SYD cheapest rtn on CX - US$2040 b4 tax
BKK-LAX cheapest oneway on ASIANA - US$770 b4 tax
TOTAL - US$4240