Here are a few things I priced out for the fun of it for early to mid September.
From.........via Cancun..............via ...
JFK..........635.14 (jetblue/maya)...657.00 (AA via MIA)
Cincinnati...543.14 (USA3000/maya)...733.00 (Delta via ATL)
St. Louis....543.14 (USA3000/maya)...657.00 (AA via DFW)
LA...........604.00 (Delta/maya).....495.00 (AA via MIA)
Detroit......629.00 (USAir/maya).....680.00 (Cont via IAH)
So, just a few examples, but in most cases, there is a cost savings in connecting via Cancun vs MIA/ATL/DFW/IAH.
The biggest cost savings appear to be where you have a direct flight from your departure airport to Cancun, which is much more likely than having one that goes direct to Belize.