Buying a second hand car/suv in Thailand is it cheaper to get one in the north or south thanks in advance

My wife and I are travelling around the beautiful southern Islands of Thailand. We have already extended our visa by 30 days at the very helpful Phuket Immigration Office in Patong.

We need to extend it again and know that we have to leave the country and then return to be granted another 30 day extension.

Our question is can this be done between Koh Lipe and Langkawi travelling by return ferry and staying in Langkawi for 3 nights..?

We would appreciate any advice…