I took the train back- odd as that might seem- which wasn't cheap, and took nearly 24 hours! But it suited me, as it gave me a long chance to unwind, and in a sense, say 'goodbye' to the walking pilgrim lifestyle I had lived for nearly three months.The daytime train through Spain retraced some similar landscapes, though not the exact route, that I had walked through. Then I had a couple of hours wait at Hendaye before I boarded the night train to Paris.
Some young ones I met found the cheapest option. They went overland to Porto, then caught a flight out from there that was far cheaper than any of the options they found from Santiago.
You can fly out of Santiago de Compostela on Ryanair to either Madrid or Barcelona for as low as 6 euro plus fees, etc. You can catch another Ryanair flight to Paris but it is Beauvais airport in Paris for another 14euro plus fees, etc. If you want to go to Charles de Gaulle airport in Paris, you can catch an Easyjet flight for as low as 29 euro plus fees, etc. The prices really depend on when you fly and what is available at the time you book.
I have to do the same thing in September to catch a flight back to Canada!
Watch the luggage restitutions with Ryanair, the staff do not “bend the rules” for no one. Plus each flight is booked on a non-connecting basis so a delay on one flight and that is it. Pay for the second one all over again, at a higher fare.
An alternative is to fly to BCN and up to Paris.