Train isn't exepensive, if you see at the good moment you can find good pirce, already find a "aller-retour" from Metz to Montpellier only for 30€ in 1st class TGV ^^
and train is less expensive to use a car (gazoil is exepensive, highway tax is exepensive, ect...)
But I see 31st march is over, so we are a little be late to answer xD hope you find an help from somewhere else