American/Oneworld or Delta/SkyTeam airlines have ticket prices under $500 round trip for San Francisco or San Jose to Frankfurt, Germany. These low fares are available from mid-August to mid-May. American Airlines has tickets at $521 all the way into May 2018, while slightly lower fares are available through Momondo.com search for Oneworld and SkyTeam itineraries.
American Airlines flights with OmegaFlightStore.com ticket
San Francisco SFO – Philadelphia PHL – Frankfurt FRA – Charlotte CLT – San Francisco SFO
FlightsMojo.com ticket Delta operated flights ticketed as KLM codeshares
San Francisco SFO – Detroit DTW – Frankfurt FRA – Atlanta ATL – San Francisco SFO
For $569 you can fly Air France with transit layovers each way in Paris of 20+ hours.
San Francisco SFO – Paris CDG (20h 40m layover) – Frankfurt FRA – Paris CDG (20h 35m layover) – San Francisco SFO