I want to know the mechanics of how the SNCF increases their TGV fares as the departure date for a certain TGV trip approaches (or the train is getting fully booked).
That question may be too broad for the stack exchange format, so let me focus on the following well-delimited question which is inspired by what I read on this TA forum:
Are the fares for flexible tickets (is that TGV pro or TGV leisure?) for a given TGV journey fixed in time? Second: is -at any point in time- the price for a regular ticket lower than the one for the flexible ticket?
Feel free however to digress on other aspects of the mechanics behind TGV ticket appreciation if you have useful and relevant information.
As secondary additional information: I myself want to use the TGV from Nantes or Angers (I don't know yet which one) to Brussels somewhere during the weekend of 5-7 september 2018. I guess that's a local low-season for tourism and train occuations.