The best way to have a cheap trip in France is to share a car.
The train is absolutely expensive.
For train you can check at : http://www.voyages-sncf.com
For car sharing : https://www.covoiturage.fr/
Maybe you could register yourself at : http://www.nautiljon.com/
And write a topic about your needs (french forum, but I think you could get some help from some people)