Per person 150-200$ are sufficient if hotels payments are already done... Its better to carry cash in dollars. Cards are not accepted everywhere, also it attracts transaction charges and exchange rates are quite low.
Best and cheapest option is Forex card. it gets unloaded also on your return without much deduction whereas currency is expensive because of deduction. credit card turns out to be most expensive. avoid it
Thailand is pretty cheap of you are going to eat street food or in food courts. since you will be there for about 9-10 days, I would say be on the safe side and carry 1000 USD and maybe a credit card.