If you are traveling just once in Thailand it does not matter so much, the costs of using an ATM (Automatic Teller Machines) to get cash money will just be a small percentage of you overal costs. But when you are traveling here for many months, traveling many times to Thailand or are living in Thailand then it might be useful to realise that using a ATM in Thailand with a foreign bank card costs significantly money.
First of all Thai banks charge you money for using the ATM, between 180 and 220 baht per withdrawal.
Tip : Aeon ATM's charge 180 baht!
Then they try to trick you into using local conversion rate for the transaction, what will also result in an unfavorable exchange rate for you. This might costs you about 1 baht per us$ or euro.
Then your bank in your home country might charge you a fix amount per transaction plus a fee of a percentage of the exchange rate.
I did just take 13.000 baht from an ATM machine what and that did cost me in total 14 Euro! I mean just the costs of getting the 13.000 baht.
The maximum withdrawal from an ATM is 20.000 to 30.000 baht and it depends on the daily limit of your bank card.
Also be aware that using a foreigner debit card or credit card to pay things in Thailand will costs some extra money. How much depends on your bank.
If you are traveling many times in Thailand it might be a good idea to open a bank account in Thailand and transfer money from your foreigner bank account to your Thai bank account. But also here be aware about the costs. Most times it is cheaper to transfer "big" amounts of money. Also the costs can differ from bank to bank. Also not all banks use the same exchange rate.
Lowest cost of getting cash money in Thailand would be taking cash money from your home country and change it in Thailand voor Thai baht. But be aware your country might have restrictions about exporting cash money. And actually any country where you are in transit on your way to Thailand. And also Thailand has restrictions on the import of cash money. Besides that there is a safety risk with carrying a lot of cash money.