I used an AEON ATM yesterday (16 April) in Bangkok with a U.S. Visa debit card, the fee showed as 150 THB.

I haven't stayed up to date on the fees at other ATMs, using either MC or Visa, but think they may be in the 180 - 200 THB range. You should get notice of this before finalizing your transaction.

A neighbor was complaining recently about using MC at Kasikorn, or Krungsri (Bank of Ayudhya), and getting hit with a 200 THB charge.