The general rule is:
For basic common consumeables like air filter, oil filter, belt tensioner, suspension bushings, etc. the price difference is so small, that they are likely to be more expensive in Singapore due to the GST.
Those replacement parts that are rarely needed like suspension height sensors, wheel speed sensors, interior control panels, instrument panel, power window components, electronic systems (EWS, ABS, LCM) are significantly cheaper in Singapore despite the GST. From my checks with Eng Soon, some rare parts can reach 20% cheaper there.
Oh yeah, and TWS oil is almost half price down there.
But it's best to do your own comparisons. Call up to check before making any decision where to get it.