Malaysia Airlines and Singapore Airlines together operate something along the lines of 10 daily roundtrips on the route, and have joined forces to offer a so-called "Shuttle" fare. You buy a stand-by ticket and fly on the next flight on either Malaysia Airlines or Singapore Airlines that has an available seat. There's usually no way to tell in advance which of the two it's going to be.

Details here.

Note that it is not recommended on Fridays, Sundays, Public Holidays or days framing Public Holidays.

A pre-booked ticket on Singapore Airlines SIN-KUL-SIN, first flight out, last flight back is 420 SGD on a date I randomly picked (Thursday of next week).

Pre-booking a one-way SIN-KUL (209 SGD) and a one-way KUL-SIN (399 MYR) works out to be a little cheaper, but you have to mess about with two bookings.