Check all the prices, but your best bet will probably be Air Asia. With advance purchase they are dirt cheap on that route. KL budget terminal isn't really that bad. There are at least two bus operators that run from KL budget to KL Sentral, one is 8 ringits the other is 9 (less than 3 USDs...). I take whichever one is leaving next paying the driver directly (don't bother buying tickets as your exit immigration). It takes a little under an hour to KL Sentral.

PS. If you are a drinker don't forget to stock up on booze at the duty free store before customs. Alcohol is really expensive in both SIN and KUL.