Modern pumps are designed to be cheap to build. Much as the Model 12 has fanatical adherents, it was blown out of the marketplace over 40 years ago because an 870 was half the price of a Model 12 and shoots about as well -- at least for the jobs people buy pumps to do. Pumps aren't all cheap to make; the ones that have survived to 2008 generally are, though. The old Remington 31 was gorgeous, but would you pay $1200+ for a Remington pump today? That's probably what it would cost -- look in the receiver at all the work it took to make it fit together. It's SMOOTH, but for that money, the mass market doesn't buy pump guns that will end up in the bottom of a boat.
Also, pumps only require one barrel, and the rest of the innards can be made mostly of cheap stamped metal. Very little precision machining is required; the barrel has to fit right.
Guns with two barrels and a quality break action require extensive precision machining. They also have two barrels. Buy a second barrel for a Wingmaster and it gets more expensive. Now consider the hand fitting required to make the barrels shoot to the same point 75 yards out. Nobody has figured out yet how to automate this.
Ruger tried to build a SxS for $1995 MSRP. They failed. There was plenty of demand, but Ruger couldn't make them for the price. They wanted to, and they had plenty of experience with the similar Red Label O/U, but they couldn't do it. The price is now well over $3000.
