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how much money do I need for 2.5 weeks in Thailand? Where's the best place to exchange money?
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There are exchange places EVERYWHERE. Little booths run by banks that are more common than Starbucks locations in any major city in North America.
There are also ATMs EVERYWHERE. Look for the "PLUS" symbol:
At any one of them, and make sure your debit card is on the network. You could also withdraw money from your credit card, but seriously, why?
For under a month, I'd say $1000 (US) would keep you very comfortable as long as you don't need to stay at high end chain hotels and don't plan on doing a lot of shopping.
One of my my last trips I spent almost $1000 on clothing and shoes alone, whereas on another visit I spent that on accommodation, transport AND food for an entire month. You can be extremely flexible with your budget.