Nobody who is sane pays for Thai Airways "C" class at the published fare!!!

Usually, if you call a consolidator (usually asian travel agents), you can find special fares for Thai Airways Business Class to asia.

Thai Airways like their competitors have to file their tarrifs with IATA, and usually these fares are the same. IATA does not allow any of its member airlines to sell directly to the public at prices lower than what are published. The only way to get around this would be for the airline who has an inferior product (or not enough customer buy at the full fare) to offer "net fares" or "commission overrides" to their preferred agents (who in turn sell it to the customer at a lower price).

Note that the above only applies to international fares and not domestic fares where the airlines are free to set their own prices.

The reason why fares on carriers such as CI, BR etc. are lower is because usually when you combine the fares for US-Taiwan with Taiwan-Rest of Asia, they price out lower than the US-Rest of Asia fare alone.