@Phil *****
Car wear and tear is not really even something you can calculate, it's a mere 600km trip. Fuel is probably like 1500 Baht and if we're talking just a border bounce it's an easy day trip. The arrival pass should be US$10 or at least that's what it was last time. Then of course the cost of the boat plus parking but overall well below 3000 Baht.
If you fly Singapore Airlines from Phuket to Singapore return, it will be a lot more than that, probably 7000 and realistically, no one is doing a single day return by air.
Also, Singapore is expensive. Even if you stay one night you'll be spending 7000 Baht a night on a hotel room, unless you don't mind staying in a dump, which will still cost a whopping 2500-3000 Baht. Food is now more expensive than Australia.
So I think the Kawthoung trip from Phuket by car will surely be way cheaper. Even flying a low cost carrier will cost you more just for the flight.