About 20 pounds round trip on the night train. Takes about 8 hours. You get a bed on the train. My advice would be to stay in KL for a few days (you might like it, who knows?), then take the train to Singapore for a few days.

While you are in KL, if you want a view of the city, don`t go up the Petronas Towers, you can't go to the top. Go up the KL tower instead.