Hey, Cheshire....we're in Palm Springs. Fares are outrageous from California to Belize right now. We're going through Cancun (on our "sayonara tour") and we'll end up leaving out of Miami to Cancun for $169 one-way.
Here's a couple of other options, price-wise:
1. Spirit Air from L.A. to Ft Lauderdale then down to Cancun. Long trip, but great prices.
2. Express Jet from Ontario(CA) to Austin. Vivaaerobus from Austin To Cancun. (about $200-300!!! pp)
3. Alaska Airlines - L.A. to Cancun non-stop (but pricey).
When are you heading down?