American Airlines/Oneworld and Delta Airlines/SkyTeam battle for San Francisco and San Jose to Frankfurt, Germany with round trip tickets at $450 to $499 for travel mid-August to late October 2017.
I had difficulty getting low fare $465-$479 dates found on Google Flights to price these tickets through direct airline website links.
Searching Google Flights for low fare dates and then checking Momondo.com revealed even greater discounts.
Momondo.com OTA search Oneworld flights
San Francisco SFO – Dallas DFW – Frankfurt FRA – Helsinki HEL (overnight) – San Francisco SFO
Momondo.com OTA search SkyTeam flights
San Francisco SFO – Atlanta ATL – Frankfurt FRA – Paris CDG – San Francisco SFO
$466.78 Wed Aug 16 – Sat Aug 26 (FlightsMojo.com)