For your trip to the Cévennes, it is easy to prepare your trip reservations since the SNCF will reimburse or exchange all tickets before departure. The nearest TDV station is in Nîmes. I invite you to book your trip ONLY to this station (by choosing Nîmes-center). Apart from there, or a carpooling will be very often possible or then a TER for Génolhac station to buy specifically on: https://www.ter.sncf.com/occitanie, because very often there are tickets at 1 €.
Either a total cost of your trip, from Paris, to 20 € (19 € ticket and 1 € TER) in Ouigo, or 42 € in TGV Inoui.
