There are 3 networks available in Thailand; True, Dtac, and AIS. True is probably the cheapest but 1-2-call (AIS) and Happy (DTAC) give the best coverage. Starting rate for a sim card is THB 59 - THB 99 and the refill card varies depending on how much usage that you need. Especially, DTAC offers to lend "THB 30" for emergency which will be deducted next time when you top up.
