To be honest, I don't know. It's hard to compare for the US domestic flights, since you have to decide on comparable routes.
That said, flights throughout Asia at distances similar to US domestic flights tend to be reasonably cheap.
I decided to make a comparison:
Fly Singapore Airlines from Singapore to Hong Kong. This will cost you 225 USD in economy.
A domestic US flight of the same distance would be San Francisco to Albuquerque on United Airlines. This flight will cost you 468 USD.
So it seems to me that Singapore Airlines is able to provide better quality at much lower cost. Perhaps the US airlines simply don't have to be better because there is no one that shows it can be done?