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Boulder airport WBU is a bit small, use the Denver airport DEN and have to make the trip (about 20 mins from memory) from Denver to Boulder. only 32 miles anyway.
All depends on airline you want to fly... northwest will fly DEN-MSP/SEA-NRT-SIN and cost about $2200-2300 SG return.
Singapore airlines do it DEN-SFO/LAX-ICN/TPE/NRT-SIN or similar cost about the same, if you want oneworld option sif SG 3600 approx CX (Cathay Pacific) can get you there DEN-SFO-HKG-SIN.
I am sure there is so many options it will drive you crazy just investigating them but thats a few, there is even cheaper like EvaAirways but i just listed the ones you may be most interested in, USairways also flies this route, and ANA (japanese)