I don't know how reliable this site is, but I thought I would share these links:
Chicago to Belize City, BZ from $348 - Save $110 http://www.farecompare.com/search/click.html?market=CHIBZE&date=2007-01-11-11-00
St Louis, MO to Belize City, BZ from $348 - Save $251 http://www.farecompare.com/search/click.html?market=STLBZE&date=2007-01-11-11-00
It says that these fares are "new" and may not been added to online booking sites (Travelocity, Expedia, etc). I have not been able to find these deals yet but will keep looking. If anyone else find anything, please let us know!