It is not possible to buy mid-week economy tickets with SAS to NYC. I have tried endlessly to find it. If you search with a saturday outbound you will most likely have cheap economy tickets available on the dates where you previously only had Economy Extra. At least this is my experience.

First I was bewildered by this, then I got angry, then I changed airline for longhaul flights. But I hear they still fill their planes so I guess it works, wich is admirable.

The example with BA, above, is not quite relevant though. Prices are about 20% off from select destinations. While this is more about trippling the cost of a mid week travel compared to a Sat-Sun stay trip.