Fares shown below have been corrected. I originally inadvertently priced two tickets (one adult fare, one senior fare).
It's frustrating not to be able to hit the SNCF website and give you real-world examples, but as a general rule same-day TGV fares are quite high. As is often said on the forum, most of the people paying those fares are business folks with expense accounts. One-third off (or whatever precise discount the senior card confers) is likely to be a good deal of savings.
Yes, in situations allowing purchase of tickets way in advance, the cost of a non-refundable ticket will be a lot lower than a regular ticket with the senior discount. But that's not your situation on the day you land in France. It is extraordinarily unlikely that any deeply-discounted promotional-fare tickets will remain available for an afternoon train that same day. Your choice is going to come down to whether you pay full fare or full fare less the discount you'd get from the senior card.
Trainline, like RailEurope and some others, is a third-party reseller. It used to be considered a reasonably good source of train tickets. That may or may not still be the case, because a third-party seller can change its sales procedures at any time. There are at least four ways a reseller can cost you extra money:
- It can choose not to display the cheapest fares (RailEurope used to be famous for that, I believe), effectively forcing you to buy a more expensive ticket.
- It can mark up the price of the tickets; that upcharge might not be clearly disclosed.
- It can add a service charge.
- It can display prices in the purchaser's home currency rather than the currency of the country where the ticket will be used, using a conversion rate that is quite different from the interbank rate (which is close to what your credit card will use).
You cannot do anything about #1 - #3 (other than by trying to buy from a different source). You may be able to avoid #4. Trainline.com allows you to specify your language and your currency. Choose the currency of the country where you'll be traveling (euros for France).
This is what I'm seeing on trainline right now; I specified a passenger above age 60 (no card), but I don't think that normally gets you a discount on a TGV. I ignored the trains before 10 AM, which will not work for you. If you wait until you arrive in France and buy a ticket then, I'd budget for the highest fare shown for any date below.
5/25 (tomorrow): one-way fares from 70 to 116 euros per person.
5/29 (Saturday): 84 to 127 euros.
6/3 (Thursday): 79 to 102 euros.
6/6 (Sunday): 85 to 111 euros.
6/9 (Wednesday): 41 to 93 euros.
6/13 (Sunday): 61 to 111 euros.
6/24 (Thursday): 106 to 169 euros.
7/22 (Thursday): 41 to 85 euros.
8/5 (Thursday): 46 to 79 euros.
8/19 (Thursday): 41 to 73 euros.
8/26 (Thursday): 35 to 79 euros.
8/29 (Sunday): 41 to 79 euros.
August 29 is the last date for which I see multiple departures. For September most trains aren't listed yet. Early September departures of the 7:17 AM train are going for 20 to 26 euros at the moment.
You can see the pricing pattern. Fares creep up as the travel date approaches, because other travelers buy the cheaper tickets. The difference between buying as far in advance as possible and buying on the day of travel seems to be about 134 euros per person.
I don't know when the September schedule will be loaded, or whether the entire month will show up all at the same time (perhaps on May 31 or June 1??). I don't follow TGV fares because my rail trips are usually shorter and thus on cheaper, regional trains. Perhaps a knowledgeable poster has a good idea how low the initial price of a Paris-Bordeaux TGV could be if you bought the ticket the day it went on sale. It looks as if you might snag a 35-euro fare by buying when the tickets are initially released.