Take the coach from Singapore to KUL. It goes from city centre to city centre and the general time you save with transfers will put it may be an hour max. slower than flying. It'll also only cost 10 or 20% of the air ticket.

It's been discussed in this forum before, so I hope you can find it but there's one outfit runs a coach with 20 sleeper (i.e. business class on plane) style seats.

Air Asia then fly from KUL to LGK - fares tomorrow are RM90 which must be about US$30.

As for the return, you could either do the same coming back or fly LGK-KUL-JHB which is Johor Bahru, just across the border from Singapore. Air Asia or MAS can do this for you. You can then get a bus back to Singapore from there.

One final option would be to look at the one way fare from LGK-SIN - it will likley be 30% or so cheaper than the one way in the other direction.

Long distance coaches in Malaysia are however of a very high standard, so if you really want to save the cash, do look at this as an option.