These low prices are available from many locations. As you see on this calendar, you can get even cheaper tickets flying out of New York (and can lower that to just $225 by purchasing in Swedish Krona). There are also tickets starting at $319.40 from Los Angeles and Las Vegas, $372.50 from Boston, and $374.60 from Fort Lauderdale.
As always, prices change quickly, so get the tickets soon if you want them. Also, note Norwegian is a budget airline that can charge for all the little amenities that don't involve going to the bathroom. To really make it a cheap trip you'll have to do a carry-on only.