Previously mentioned Megabus seems to have pulled out of everywhere west of the Mississippi.

Another option- Karmel Shuttle is used by my employer and they are very good. Take one from LAX to "East Irvine", where you can catch Amtrak to San Diego. That's probably the lowest fare for the distance...you could also take Karmel to D'land connecting to another bus, or to the Anaheim Amtrak station.