Market prices apply to TGV (TGI) Brussels ⇔ France:
A seat reservation is compulsory and included in the ticket.
The earlier you book, the more likely it is there will be inexpensive tickets available.
The lower the ticket price, the more conditions for changing and cancelling apply.
TGV (TGI) Brussels ⇔ France operates with 'Souplesse Tarifaire': the ticket prices are variable and depend on the time of booking and how crowded the train is expected to be.
You can obtain a ticket price by entering your journey details in the What is your destination? section at NS International.