Check out Ryan Air's website for very cheap flights. I spent hours (!) trying to get the cheapest/ best connections in September. I switched between Google Flights, Expedia's +/- 3 days option and SkyScanner. I'm going from the mid-US through NYC (I think it's LaGuardia), then Madrid. I'll prob fly from Madrid to Bayonne/ Biarritz via Ryan Air. It was cheaper to fly into France, but I want a day or two to jet lag and France is expensive. Taking that into account, Madrid vs. Paris seemed to be a wash as far as price goes. I'll see Madrid for a day or so, and hopefully get to see Barcelona at the end of my trip. Flying internationally to Barcelona from the US was much more expensive.
Oh, and you might look into flying through Dublin; Ryan Air seems to originate there.
One other thing: I figured flying back to Madrid after Santiago would be easier than trying to get back to Paris or Dublin. If I can't get a flight whenever my camino ends, then I can take the train or bus to Madrid. I padded a few extra days into the trip for exactly that reason.