As a photographer working here in the Mayan Riviera we deal mainly with destination brides from the USA and Canada. We have a standard $500 deposit for all our weddings with the balance due on or before the wedding. This is approx 25% down. So 30% is not unreasonable. Just make sure your dealing with a reliable vendor and everything should be fine. Most deposits are also non refundable. This is due to the fact that once we take a booking, we start to turn down other work to reserve the date for you. Best of luck!