Just for info. We arrived in Thailand on a 60 day tourist visa (UK). This is due to run out early January. We thought about a border hop, but as we'll be on the islands not so easy or cheap.
Anyway, went to krabi immigration today and they extended 30 days from the January expiry date. Cost 1900tbh each, need a copy of passport and photo but that can be done there, I'm not sure if it was 20tbh for copy or 100tbh for both ( we had ours already)
Easy system, today was a busy day after weekend and holiday sl took us 3 hours but usually 30 minutes wait. Be aware its cash only too.
Hope it helps.