You did not mention the amount of people in your party. But if you have over ten you can qualify for a group rate thru the airlines. I always use a wholesaler which you should be able to find in your area. Years ago they received prices for the whole year but now it usually goes in three or six months increaments. Just yesterday orbitz quoted me a price of $468.00 and travelocity had a price of $418.00. However my wholesaler said that prices to Belize where scheduled to drop starting August 16th and she was able to get us tickets for $368.00.
Naturally you have picked prime months for Belize which relates to high ticket prices.