Based on the prices you mention, I guess you’re trying to book a domestic French TGV reservation. Note unfortunately only a limited number of seats are available for that price, and when those sell out the remaining seats are sold for €20 per passenger.

Check https://travel.b-europe.com/Eurail-GE/en/booking-tgv to see whether €10 tickets are still available. It seems the Interrail reservations portal is currently not able to show those €10-tickets, so there might actually be €10-tickets available by booking through b-europe or SNCF.

Prices in the interrail reservations portal add +€2 booking costs per ticket. When booking through the b-europe website they charge you €4 per booking. You can avoid those costs by buying them through the phone from SNCF (+33 184943635, dial #85 to be helped in English) or buying them at a SNCF ticket machine & ticket desk.

Note the Interrail app has no realtime information on reservation availability, and just mentions the minimum price.

Yes, these are the prices for TGV in FR on SNCF

IF you are willing to use slower trains-in many cases, but not for very long trips, this can be done on the slower regional TER trains- for free.

YOu also only pay the 10/20€ without any extra IF you are a longer time in FR anyway and do it via the purple SNCF machines-though this needs some tricks. Then you can see all trains and the extra. For not that busy days a few days advance is usually enough. Note that NO paper tickets are then given out-they also want fone+ @mail and send a mail! Modern style! Or you could even visit the counter after having seen what trains have the 10€ seats and let them do it for you=same price.