I've been checking Captain Train for days, and today for the first time I was able to book online my TGV ticket, Paris to Barcelona. Since I like my comfort, and the trip is about 6 hours, I splurged on 1st class for 109E (about $123 USD). That's a semi-flexible ticket so I have some room to maneuver if necessary. There are cheaper fares available as well, if you want second class without any flexibility I believe that was 49E.
I was quite surprised actually to see the October dates come online so early (I booked for an Oct. 9 departure from Paris). I fully expected to have to wait another 3 weeks at minimum for early October booking.