My last trip down (2 weeks ago) I booked through hotwire.com
SLC-Belize City is around $590 on AA, but the you pay departure tax when leaving, and Continental was $635
As there are limited flights into Belize, and all travel is for same day arival and a departure after 6am, major US cariers only, I thought I'd try and see what I could save
I was able to look at different dates for the best fare, and settled on a Monday departure, and a Thursday return 10 days later on Delta, via Atlanta for $476 which included departure tax