Star Alliance airlines have San Francisco to Paris round trip tickets on sale at $336 and SkyTeam at $418 round trip for summer flight departures starting August 10, 2018.
Google Flights shows low ticket prices at $427. Momondo.com pulls up online travel agency fares starting around $336.
Momondo.com meta-search – TravelMerry.com
United Airlines codeshare flights
San Francisco SFO – Vancouver YVR – Paris CDG – Zurich ZRH – SFO