Why is it considerably cheaper to fly from NYC to BCN than to Madrid ?
Probably because more people want to go to Madrid. Its that capitalist supply-demand concept of pricing
Norwegian flies into Barcelona but not into Madrid. I suspect that's the explanation: competition from a budget carrier.
Fares to Barcelona are stunningly low.
We flew into Madrid from Boston last June very reasonably and flew back from Paris to Boston--all on Norwegian Air.
Barcelona is only serviced by United, American and Norwegian Air from the U.S--and Norwegian is always very competitive on airfares on new routes.