Delta and United both have matching $275 low fares from Los Angeles to San Juan, Puerto Rico SJU for departures from late September to early December. Ticket prices are $297 from San Diego and $303 from San Francisco. American Airlines fares are about $50 to $75 more.
Delta Airlines appears to have the most available low fare dates.
Delta Airlines Los Angeles LAX – New York JFK – San Juan, Puerto Rico SJU – JFK – LAX
United Airlines San Francisco SFO – Washington IAD – San Juan SJU – IAD – SFO
These are domestic one way fares priced as low as $149 one way from San Diego SAN to San Juan with both Delta and United. You can fly one airline in one direction and another on the return to mix and match best prices.
United Airlines ticket flying Delta Airlines LAX – SJU and United Airlines SJU – LAX.
Fri Nov 4 Delta San Diego SAN – New York JFK – San Juan SJU
Sat Nov 12 United San Juan SJU – Houston IAH – San Francisco SFO – Los Angeles LAX