Heading all the way back to Barcelona is a lot of travel. What I would do is take the train from Pamplona up to San Sebastian (about 1.5 hours) and then there’s an overnight train to Paris that takes 9.5 hours with one change in Hendaye as you cross into France (about 30 minutes after departure from San Sebastian). I can’t find this train on the Spanish national rail service site, but D-bahn shows it. I’d buy your ticket upon your arrival in Madrid as a lot of these major trains in Spain fill up fast in the summer.

Use www.whichbudget.com to see which low fare airlines fly which routes. You might find something cheaper and more convenient than an overnight train.

The nearest airports would be Bilbao (1.5 hours by bus from Pamplona) and all flights to Paris will require a stop somewhere and cost over $200. If you decided to fly you could stop in Bilbao to check out the Guggenheim before your flight. If you decide to do the overnight train from San Sebastian, leave Pamplona early enough to give yourself a little time to see San Sebastian because it’s a beautiful city.

Closest nonstop flights I see are Zaragoza to Beauvais, or Santander to Charleroi with Ryanair.

Skyscanner.net found Bilbao to Paris with 1 connection for $125 — but be aware that one this itinerary, Airline B does not and will not accommodate you if Airline A gets you there too late for any reason.