Icelandair has San Francisco SFO to Paris CDG as low as $350 in December, $364 Jan-March and $450s on many other dates.
San Francisco to London shows $367 round trip on Google Flights, but the lowest fare I found is $435 through Icelandair website. Icelandair has many cities in Europe on sale for winter travel.
These are carry-on only economy light fares. Upgrade for $50 each way to standard economy with a free checked bag and 100% Saga Miles. An Iceland stopover is available with either Economy Light or Standard fare.
Icelandair San Francisco – Reykjavik KEF – Paris CDG – KEF – SFO
Icelandair San Francisco – Reykjavik KEF – London LHR /
Note: Departs London LGW – KEF – SFO