My brother always flies out of LAX with Alaskan Air. Not sure if it's the cheapest, but it's one of the few that has direct flights. I believe he just checks day after day trying to nab a great price. Good luck! (He has seen much cheaper deals flying to Detroit then Cancun, weird I know, fly all the way across the country then south, but it can save you some bucks).
Direct flights from California are with Alaska, Mexicana y maybe Frontier. You can also try some of the companies like Funjet for "air only" ticketss.
We always fly Alaska Airlines from LAX to CUN. Roundtrip is between $350 & $450. I have a friend that insists it is better to fly out of Tijuana, but that is too far from San Jose. You are better off going out of SFO or LAX.