Copa Airlines has some pretty amazing flights from San Francisco to San José Costa Rica right now. Flights start at $256 round-trip, after taxes with open availability between October 2018 and May 2019.
Copa Airlines is part of the Star Alliance and these flights are coded “T,” which earn 100% Premier Qualifying Miles on United, so this can make for a great mileage run.
To find this deal, use my Expedia link to search for flights between SFO and SJO.