SXS cost more, like other things that cost more, because they are worth it. They are worth it to those who buy them. And nothing sets the real value like the market. Try to sell a used gun and you will find out right fast what it is really worth.

Larry is right that volume make many O/U cost less per unit but no maker would build a gun at a loss for very long. Prices are because that is the max. that sellers are willing to pay.

Maybe SXS are slightly more labor intensive, maybe not.