Cheap flights to San Francisco are cheapest if you fly on the remaining days of September or on the first two weeks of October. Roundtrip tickets from New York to San Francisco on these dates are only at $306 at Delta Airlines, compared to the $348-$496 you'll have to pay if you fly in November (the cheapest offer coming from Virgin America). If you're traveling from, say, Chicago, you can fly for only $256 with American Airlines if you leave on the 22nd. However, prices are still relatively low at $282-289 if you fly from that date and the whole month of October. After October, roundtrip tickets will range from $289-481.
Below is a list of San Francisco airports that you might find useful:
Flights
