On a sample itinerary of September 15 to 20 on Google Flights, you can fly to Stockholm for €363.10 which converts to about $414.00 round trip.
Once you have a sample airfare in USD from Google Flights, head on over to the Norwegian site and configure the same flight but select Euros as your currency of choice. Adjusted to the current conversion rate, you’ll save a bundle, resulting in a final fare of $414.00.
In September, Stockholm still enjoys 182 hours of monthly sunshine, which is nearly fives times as much as you’d anticipate in the wintertime.