Occitanie TER (liO) has, I believe, 4 kinds of cheap-to-mad fares:
Avantagio Futé EUR 3, 5 or 10 by distance, off-peak trains only
Avantagio 1eur - above, few routes
1ers weekends du mois 1st Saturday, Sunday (and following Monday if public holiday) 1eur potentially on any train but quota limited, on sale from 10 days ahead only
Occitanie Rail Tour 2-6 days - above
These are all per person.
Normandie, Bretagne, Pays de la Loire, Centre-Val de Loire TER have 1-3 day 2-5 person rover tickets but some are weekend+public holiday only.E.g. Normandie weekend 2 persons EUR 20, Centre-VdL 3 days 4 persons EUR 60. Most include region-run buses (which may be sparse). Grand Est has about 5 rovers but so perplexing you need to start with the precise journeys to choose.
There are other summer-only, e.g. PACA has several 1 / 3 / 7 / 14 day rovers for 1-2 départements or the whole region but not particularly cheap, Hauts-de-France claims to have EUR 1 tickets but you have to hunt for them through the planner.
For each region, go to the relevant ter.sncf.com but check both 'Tarifs' and 'Bon Plans' as the cheap fares and rovers can be in either place.