Check Air Transat, Air Canada, and maybe even Air France. Air Canada has had a succession of sale fares recently. I am signed up for email notifications for both Air Transat and Air Canada. This provides notifications of specials and deals. Suggest you sign up also if your budget is tight.
I think it is easier to start from Saint-Jean-Pied-de-Port if you fly into Paris. But if you start in mid-March, the pass over the Pyrenees is going to be closed, so what's the point, really? You can start in Pamplona and the logistics of getting there from either Madrid or Barcelona airport are cheaper and simpler. You're planning on going all the way to Finsiterre and Muxia, so you'll have no shame whatsoever about starting in Pamplona. Just my opinion, but the movie The Way has convinced way too many people that they must start in France at SJdPP.
If you start in Pamplona, consider a very short day and stop at the albergue in Cezur Menor. That's what my wife and I did in 2013 and it provided our intro into pilgrim's meals, albergue life, and allowed us to meet fellow hikers which we did not do when we arrived in Pamplona late in the evening before. That was a mistake, because Pamplona is a great place to visit, with extremly reasonable food and drink plus things to see. Fly from Toronto, bus or train to Pamplona, spend the night and then explore Pamplona the next morning. Then begin your Camino with an easy hour and a half walk to Cezur Menor.
Buen camino!