Puerto Rico is on sale from a variety of cities with other airlines besides Spirit. San Francisco SFO to San Juan, Puerto Rico SJU is as low as $259 round trip on Delta for travel for hurricane season travel in September and October. Seattle SEA to Puerto Rico SJU is as low as $269 on United.
Delta Airlines San Francisco SFO – Atlanta ATL – San Juan SJU – Atlanta ATL – SFO
United Airlines Seattle SEA – Houston IAH – San Juan SJU – Washington DC IAD – SEA