Agreed, €54 is a bit much really!

Domestic TGVs are always €10 if you book before the limited passholder quota is sold, and €20 if you are too late and only book after that. You can take a direct domestic TGV from Paris to Perpignan, then you have two options.

Either board the SNCF-Renfe TGV you mentioned before but only between Perpignan and Barcelona (this should be no more than €15 for this distance, make sure you are charged the correct price), or take a regional train to Portbou and change to another one to Barcelona (neither of these require reservation).

You’ll also find taking a domestic TGV from Lille to Paris to be the same €10 as Paris to Perpignan, again, provided you book in good time. To get to Lille, get off the Amsterdam-Brussels IC early at Antwerp, then take another direct hourly IC from there to Lille Flandres. The timetable will tell you that you need to change at Kortrijk, but you don’t - the train just gets renumbered there because one section is separated there and goes off towards Ieper (make sure you board the right carriage at Antwerp!).

Lille Europe, from which TGVs usually depart, is a 5 minute walk away from Lille Flandres.

What I usually do is either take the Thalys for €25 Amsterdam to Paris, or I take the TGV for €10 Lille to Paris and travel to Lille by reservation-free intercity trains. 

From Amsterdam to Spain is doable in one day, I've done this twice, both to San Sebastián. If you take the early Thalys you'll arrive in San Sebastián around 7 pm. You will need a reservation for Thalys and TGV, total €35. From Hendaye to San Sebastián is done by Euskotren, which is not covered by Interrail but very cheap. Both San Sebastián and Bilbao are interesting places to visit. Between San Sebastián and Bilbao you travel with Euskotren as well. It's quite cheap and it saves you a travel day. You could then take a direct train to Barcelona, reservation €6,50. From Barcelona back into France, I'd recommend the route via Cerbère which takes longer but saves an expensive reservation.

Since for reservations the distance doesn't matter, consider thaking a long train route. For example, from Lille there are direct TGV's to Bordeaux and Marseille, which by-pass Paris. Plus, you'll save the hassle of taking the Paris metro.

Another option to consider is to take the night train from Paris to Cerbère or Latour-de-Carol. From there you can continue by regional trains to Barcelona. 

Going to Istanbul definitely sounds like an awesome journey. Have fun planning your trip!

Thank you guys so much for the awesome replies! The options you’ve provided certainly are more attractive alternatives than what we originally we’re planning. 

No probs! If you need more help with planning your trip, don't hesitate to ask!

I have not checked all of the reservation costs yet, but I will be travelling from Spain back to Belgium (so the other direction you are saying but the same trajectory). I am doing Madrid - Girona, overnight there. The next day Girona - Brussels via Lyon, cause I really want to avoid having to change train stations in Paris (tooooo busy). So maybe this is an alternatively worth looking in to?