Hi SMS
if you want to see comparative rates for THB against ather countries this is kind of handy:
Exchange Rates Table for Thai Baht, Thailand
But xe.com is more reliable and nothing beats Bloomberg or Reuters.
In Thailand in general banks and money changers dont charge commission - they just take a margin in the spread (buy/sell) - this is between 1 and 2% of the rate you see quoted on any of these sites.
Always change you money over here - and then better in the main tourist areas rather than at airports.
Perhaps you may find some unscrupulous banks try to make bigger margins (spreads) on their quotes for THB - but considering that THB only weakened 1.5% at the worst and now is a mere 1% weaker than before the coup the base currency could well have moved more than this to eliminate the potential benefit. FX moves of +/- 1% to 1.5% are within normal daily bounds.
PS: (Sober at the moment).