I was perusing Flyertalk today and came across an awesome deal on flights between Los Angeles or San Francisco and Costa Rica on American Airlines. You can find round-trip fares for under $300. You can find all the details using this link. This is about half to one third the cost of other airlines. And it’s a very low cost/mile flight so it will help bump up your EQM (butt in seat miles).
UPDATE: The deal is also good out of LGA and DFW.