Fly nonstop from San Francisco to Paris, France for $447 roundtrip on Air France for fall and winter travel.
Flights at this price open up around early November and are valid for daily travel through the middle of May 2019. You'll have the best luck finding seats at this price starting in 2019, where United Airlines begins to match the price as well.
As a warning, this flight books into basic economy so you'll have to pack light as only a carry-on, and a personal item is included. If you need to bring that bag full of fashionable designer clothes to keep up with the Parisians, you can upgrade to the standard economy fare bucket for $526. Which is better than paying the $60 each way for a checked bag, you'll end up saving $40 in the long run
As an example, on the Air France website, I found seats departing SFO on March 1 and returning from CDG on March 11, 2019. Dates in the winter are wide open, so you shouldn't have any problems finding availability. A 7-day minimum stay is required.
Need a room with an Eiffel Tower view? Here's what's available for the sample dates on Hotelwatchdog.