Within Thailand only Business Class is offered according to Expedia.

I haven't been able to find a lower return F fare than USD 166 from KUL-SIN (SQ & MH - it's the most boring route in the world; farewise.)

SIN-KUL Y return is 166 USD, but KUL-SIN F is 166 USD. If you book two one ways, you can have Y and F on the same ticket for the same price as a return Y class ticket from SIN.

This route is about the only route where tickets purchased at the airport are the cheapest. APEX doesn't exist on this route, full Y or full F - mind you, you don't really get much apart from transport and FF points.

Cheers,

Justin