Delta Airlines has a low fare of $370 to $374 beginning in late August through winter 2016 from Los Angeles or San Francisco to SJU San Juan, Puerto Rico.
Delta Airlines San Francisco SFO – Atlanta ATL – San Juan Puerto Rico SJU
$378.60 November 11 – November 18
Los Angeles LAX – San Juan, Puerto Rico SJU
$373.60 September 9-16, 2015
United Airlines is $215 each way and American Airlines is $229 each way for this route, so you can mix and match for best schedules.