Fly from Paris.
For the train from Moscow to St. Petersburg, you can take an overnight train. It used to be Leningradsky Vokzal in Moscow, but I think the name's been changed to Oktyabrsky. The express trains take about four hours, but from what I saw look much more expensive (I think it was €80 vs. €40 or less). Unfortunately the ticketing info only seems to be in Russian
; the English version of the site doesn't seem to sell tickets.
Edit: Apparently RZhD now has a train from Paris to Moscow
, which you probably saw, but the tickets are €330 for a 37-hour trip, which is twice what I saw for a one-transfer plane trip.