Upvote:4
The World Cup is held in 12 different cities which are very much far from each other, so the answer depends on which games you are planning to visit:
Kaliningrad is the easiest - you can take the train (or fly on a low-cost airline) to Gdansk and take a bus from there to Russia. This would be cheaper than flying through Moscow and not too inconvenient.
St. Petersburg might be viable if you fly to Estonia or Finland first and then take a bus/train to Russia. From Helsinki the train costs ~30 EUR and takes 4 hours. From Tallinn there's an 8 hour bus for 20 EUR.
If you do visit St. Petersburg first, it would also be viable to take the train to Moscow from there. For 50-100 EUR there's an express train between the two cities, taking 4 hours.
In every other case you are much better off flying as it would be cheaper and a lot faster. Unless flight tickets prices increase astronomically there's no way you would save money by traveling over land.
Apparently Russia promised to provide some form of free public transport for fans, however the details are currently too vague to provide a proper answer. If you're willing to wait, hold out for an official announcement on how the system would work.
Upvote:7
There are several possibilities:
You can travel from Paris (or Nice, or some other countries) to Moscow by train - Official Russian Railway site. It will take you less than 40 hours and I doubt that it would be cheaper than flying. And you might need to have a visa for Belarus. There are also trains from Helsinki to Saint-Petersburg.
You can travel to Saint-Petersburg by bus from some German or Polish cities (link).
There are ferries from Baltic countries, Sweden and Finnland to Saint-Petersrburg.