Just check the average SINKUL fares in Jan and Feb (excluding the Chinese New Year long weekend around 7 to 12 Feb). Most flights have a base fare of $0. Others are $2. the morning and evening peak flights are $10.
Jetstar Asia's Chicken Rice is SGD12. Tiger's is SGD10 (from the same caterer). Air Asia's is about SGD4 from their KUL caterer.