I like to start at the TGV website because I can see the best prices in a 30 day period or during a single day. SBB makes you pick the day and time then after clicking through, they give you the price in chf.
So, for Sept 16 at noon, I see SBB price of 41 chf for the 12:34 train. If I go to the TGV site, they quote 39 euro, which is about the same price given the exchange rate (actual .35 cent cheaper because they round to the nearest even number). However at the TGV site, I can see that if I choose the 16:34 train, I can buy it for 29 euro. So then I would buy that one at www.capitainetrain.com.