Every day I go into TACA's website to check on fares from LAX-SJO for a particular weekend in April I'm planning. When I first started checking it out, the fare was $386 r/t. Then it went to $353, then $337, then $348, then $337 and now today, back to $348! I'm not kidding. The flights are not full either, not even close according to
www.expertflyer.com 
I know the differences are not great, but why do they fluctuate daily when the flight is empty? U.S. airlines fares fluctuate daily, but this is primarily due to supply and demand...correct?
Thanks!
Ciao!