SNCF (French railways network) has special fidelity programs for frequent travellers, most of all if you're going all the time to the same destination.
http://www.programmefid.com/static/vsncf/card/fr/pp_caracteristiquesCartesCoAbo.html?rfrr=Loisir%3ATrain%3APassTgv%3AAccueil_body_Consulter%20les%20conditions%20de%20vente
Forfait season ticket (travel in France): Do you travel daily or almost daily?
With Forfait you have unlimited travel. Simply pay for your seat reservation. There are several types of Forfait season ticket available:
Fixed journey: after you have bought your first monthly card you can then choose between the weekly or monthly option;
France-wide (excluding transport in the Ile de France region): monthly season ticket;
1st or 2nd class
The fare varies according to the journey, the class and whether you are buying your first season ticket or renewing it. You can buy a Forfait season ticket:
At your Boutique Rail Europe
From a customer sales agent at a station, SNCF shop or approved travel agency in France
Fréquence season ticket (travel in France): Are you a frequent train traveller (more than two return journeys per month)?
Buy a Fréquence season ticket and enjoy a guaranteed 50% discount on TGV Pro 1st and 2nd class fares, on a specific route or France-wide. Fréquence also gives you access to all the services included in the TGV Pro fares, depending on the comfort level chosen.
The ticket comprises a personal travel card which is valid for 3 years and a Fréquence coupon (price determined by the validity of the coupon: 3 months, 6 months or one year), the journey (specific route or all SNCF lines excluding île de France services) and the class.
You can buy a Fréquence season ticket- At your Boutique Rail Europe
From a customer sales agent at a station, SNCF shop or approved travel agency in France
More information on our range of special Pro fares and services.
http://help.tgv-europe.com/en/fares/season%20ticket-frequency?rfrr=WW%3AAideFaq%3Atarifs%3Atarifs_pour_les_pros_body_SNCF%20Fréquence%20travel%20card
Unfortunately, it's not possible to travel by coach which would be cheaper.
Fast trains are expensive. Bordeaux or Biarritz are far from Paris.
Try to book now - asap - your tickets for July as during summer holidays it's difficult to find a seat.