$579 - Direct Flights to South Korea from SFO
This is the one of the cheapest flights to Asia from SFO that I have found in that last two months.
Flights are on most days in September, regardless of day of the week. It seems that Singapore Airlines is operating a single direct flight each way between San Francisco and Seoul.
United also offers cheap flights to South Korea, but there flights are typically one-stop, and cost 200 hundred dollars more.
Direct flights between South Korea and San Francisco are between 11 and 13 hours.