I'm sure it'll be at least 10x the cost of flights. Train travel is just too expensive.
If just you look at flight vs train then sure the flight will win. But the train goes from the city centre, and includes a night of accomodation. If you look at taxi + flight + taxi + night at hotel - the train will be attractive. If we correctly price externalities (i.e. environmental cost) then the train should win hands down!
A train that leaves Paris the day before at 1347 and arrives at 0655 the next morning (current schedule) isn't as useful, unless what I really wanted was to spend all day in Copenhagen instead of half a day.