Google Flights shows prices around $400 to $450 round trip for San Francisco to Paris CDG on United Airlines and Star Alliance airlines during Christmas to New Year’s week, starting December 24, 2018. The catch is cheap ticket prices require staying until Tuesday, Jan 8.
Even more flight options with better looking itineraries are available from Dec 27 through last days of 2018 for USA to Europe flights.
$393 round trip ticket on United leaving Dec 27 SFO to Paris with return Mon Jan 7 ($500) or Tue Jan 8 ($393). Dec 27-Jan 8 is a 13-day trip. There are several $400 low fare date options from Dec 24-31 with returns Jan 8 or later.
United Airlines San Francisco – Paris (UA nonstop) – Munich MUC – SFO (Lufthansa)