Your problem is the departure point and discounted seat capacity. Sacramento is not a hub airport and has little competition. SFO and OAK will have better rates, in that order.
From the west coast...
Continental flies through IAH (Houston).

TACA, the airline of Salvador, goes through LAX (Los Angeles) and SAL (San Salvador).

American goes through DFW (Dallas) or MIA (Miami) depending on the day of travel.

You can also take circuitous routes and connect on several airlines through Mexico, Guatemala or Honduras to get there but that is probably more complicated than you were hoping for.
www.flycheapoL.com has had good rates in the past. They seem to be discounters for Continental.

Good luck. I usually pay no more than 490rt from LAX or ONT in southern California with all taxes included (except Belize Departure tax).

Just got an E from Bestfares.com, they have travel for the Holidays between 300-400 roundtrip to Belize from all over the US!!!!! WE may be there for New Years eve!!!!
Thanks, Rick

PS. email us and we can give you the info. from where!!! Rick

hunters and seashell: good points. I will try different days. I know that Sac is not a hub and it is a pain. They do not even have international flights yet and they are the capitol of California. Kind of strange! However, it is so convenient to fly in here and only have 20 miles to drive home! Anyway, thanks for the information.

Hey ya all, try this and see if it helps!!
http://www.bestfares.com/travel/desks/story.asp?id=646

This will give you the updated fares from across the states...

Hope this helps... Thanks, Rick