Krabi aint that far from Singapore. No direct flights there so far (Silkair was supposed to come out with one in say April but have not heard any word from them). If there is a flight there, it would be from Singapore to bangkok and from bangkok to krabi. total maybe 3 hours and cost would probably be ard USD$200.
My advice is to fly to Phuket from Singapore (USD$150) and take charter a mini bus to drive you there. the flight to phuket is about 1 and a half hours but the ride from the airport to krabi is close to 3 hours. Saves a bit of cash if you could find extra people to share the mini bus rental. Do not take it from the airport operators, go outside and find a local there to drive you. Remember, always bargain, and they see a white guy, they always try to rip you off. Email me if you need more info on thailand. was just there during the first week of march. wanted to go diving but could not squeeze it into my limited budget,