The production cost of a double-barreled shotgun doesn't depend on the plane in which the barrels are joined - that's what I've heard from engineers employed at a gun company.

The dominance of O/Us on the clay ranges is the product of evolution and survival of the fittest. Even in the '60s, at Trap and Skeet World Championships, about 30% of participants used SbS's, 10% - repeaters, and the rest 60%, with O/Us, mostly took top places. The O/U won the clay world because more people were winning events with them. (yet it all was probably just because you don't burn your hand as so can shoot more rounds, I think ;))

Russian Hunting Shotguns