Exchange rates change (at least) daily, so we have found the following Website useful for determining the official government exchange rate for the Mexican peso, especially when choosing a bank or money exchanger at which to convert dollars:
Hotels usually provide the lowest rates, but if you're converting a few hundred dollars, relax; no matter where you convert you'll probably be within plus or minus 130 pesos – about the price of a baseball hat or t-shirt. That said:
1) Debit (ATM) cards will provide established interbank-rates, but depending on the card (and the associated bank) you may be charged fees. We like the bank ATMs in Santa Cruz and La Crucecita (rather than the airport) because they tend to have lower fees and allow the larger withdrawals. We can usually withdraw at least MXN$6000 per day with minimum fees and at the highest available exchange rates.
2) Azteca bank, with offices at Elektra near Chahue, will exchange at fair rates. You may need to bring your passport.
3) Most other banks in Huatulco will not exchange dollars if you do not have an account at their branches. Hotels tend to provide inferior conversion rates.
We always advise our bank about our travel plans and have not had a problem.
Hope this helps. Your questions are always welcome.
Casa Hoffa (South), Bahia Chahue, Huatulco, MX
Casa Hoffa (North), Fire Island, NY USA