When you buy Thai baht the amount you pay will come down to both the commission or conversion fee, and the exchange rate being offered by any provider. Currency exchange services and banks often add an extra fee in the exchange rate being used, which pushes up prices without you realising. This means the cheapest way to buy THB is often to use a travel debit card or a specialist multi-currency account to spend and make card withdrawals with low fees and the real exchange rate.