Ouigo is an independently run subsidiary of the French national rail company SNCF . It started in April 2013 between Marne-la Vallee (the Disney station near Paris) and some points south east. A certain number of seats are 10 Euros and go up as they sell. Almost always though, you will get a reasonable price. I paid 15 Euros for a fare from Paris-Marseilles in June. Kids 11 and under are always 5 Euros. Please look at the Ouigo site for more information. Below that link , are my tips on buying and what to look for.
I learned a couple of tips on my trip this past June that I'd like to pass on. First thing, before the booking process, have a French cell phone number handy. A friend's, business, hotel, etc. They only except French cell phones. The rest of the booking process takes a few minutes. Use Google translate to help if you can't figure out the French. Here's what Ouigo says on why they need that French Cell phone number.
"Your number is necessary to inform you in case of hazards on your trip, it will be used only in this context. Service available only for mobile phone numbers in French format (10-digit numbers beginning with 06 or 07)"
Second tip -
Buy an electrical outlet for 2 Euros, whether you need it or not. They put me in a top compartment with only 8 people because most people did not spend the extra on the outlets. I didn't need an outlet; I just gambled that might be the case. There are some seats on the train with rows of three and they might not be as comfortable if you happen to get assigned one of those. All the electrical outlet seats I saw were only rows of two. Not certain that's always the case, but that's what I observed.
A thing to keep in mind is, they don't E-mail your tickets until 4 days prior to your departure. Copy the info you get online just in case. If you forget to print the tickets out, they will charge 5 Euros at the platform with the reservation number.
When I just checked prices on Oct. 8, 2013, tickets for November mostly were still only 15 Euros each way Paris-Marseilles. As you can see, bargains are usually available.
Looking forward to hearing your experiences.