I think the best way is to go to Priceline(
www.priceline.com )
From my personal experenice, Priceline customer can set a price
and after it is sent to the Airline, if they think it is ok, they will let u to
buy the air ticket with that price. However, if they think it is too cheap, they will set a price to you, it will be not very expensive as well.
From my experience, I order a air ticket in the summer holiday
from Hong Kong to Signapore, by United Airline, the air ticket
is jus $600-700 (not include airport tax).
Hope I can help u with the above information.