The reason they are so expensive is simply a Marketing tactic. There may be a slight difference in manufacturing costs but not enough to substantiate the high price. O/U and SxS's are luxury guns, in the hunting world they are classics. Same reason the automotive industry has reverted back to the classic body styles. There is a demand for the classic feel. With an increase in damand there will be an increase in price. This is also why you will often see intriquite detailing on the receivers of these guns. It's a low cost opportunity to increase the look of the gun that will further justify the higher price.