From hatyai you get a taxidriver to drive you 1000 baht. One way. Probably you can bargain him down to 1500 and do the whole thing roundtrip with him. 2-3 years ago i got a taxi for 1000 one way, but i slept the night in sadao then. But anyway i think that is the pricerange you can expect.

I know an incredibly friendly and safe taxi driver in Hat Yai, he drove me to Samui once. I also made the Hat Yai-Sadao return trip many times with him, for 600 Baht (first 500, then the gas went up). He's well known at the border so everything smooth on both sides. Only thing is he hardly speaks english. Anybody interested PM me for his number.

Form Had Yai there is a mini bus. Very regular. The last time I went it was 40 baht. I used to go to Pedang Besar. Cross over and visit the duty free shop on the way back. That was about 4 years ago though.

yep did that,its now 45 baht,great value.very easy run through,stayed 2 nights had yai,had a good break.