Avianca has round-trip flights from Los Angeles (LAX) to San Salvador (SAL) for $406-$437, either NONSTOP or with a connection. Flights from last week of March to June and also August and September. Weekends available.
Note: It may be possible to get these for as low as $391 by picking a route with a connection. You’ll probably get a better value by paying a bit more for the Nonstop flights though.