United Airlines still has its nonstop LAX to Singapore flight on sale for $515 round trip for travel to May 15. Xiamen Airlines has an even lower ticket prices at $449 to $499, but require one or two stops in China each way.
Ticket prices from San Francisco and San Jose are mostly over $800 round trip. You can fly on a separate ticket from northern California to LAX for a cheaper flight to Singapore.
United Airlines Los Angeles LAX – Singapore SIN – LAX