‘Cost per seat’ is not a standard industry metric and is pretty meaningless. Cost per seat-km makes a lot more sense.
However I was surprised when I looked at the relative seating density on Norwegian and SAS (surely the easiest way to differentiate cost per seat). On their 738s they are exactly the same – 186Y. Of the ‘legacy’ carriers I think only Aer Lingus packs ‘em in like that.