There is some logic, but it’s not always obvious!
The TGV reservations are either €10 or €20, depending on availability: €10 for the first few reservations on each train, then €20 for the rest of the seats.
Then there’s a booking fee. €2 per ticket if you buy it through Interrail, €2 at Rail Europe (although €0 if you bought your pass through them) and €4 per basket at B-Europe.
The prices shown in the Interrail search are usually ‘from’ prices, without live availability. Rail Europe & B-Europe are showing live prices.
Conclusion: the reservation will cost €20, wherever you buy it from, plus booking fee.
There’s not a massive rush, but it doesn’t hurt to book it now. since these trains can fill up. You can cancel and you get the reservation cost back (but not the booking fee). Rail Europe handles cancellations very well. Interrail less so.
If you find any availability at €10, it’s definitely worth buying now.