Master,
I'd say the savings comes in the finishing.
I guess it's easier, faster, and cheaper to finish the barrel section on a lathe with minimal machining for the locking lugs, and do the complex machining on the chamber section.
It it were cheaper to do the barrel in one piece, I don't think we'd be seeing 2 piece barrels, as it probably takes more machinery.
It was also my impression that the 2 piece barrels aren't welded, the breech section is heated and then shrunk onto the barrel shank.
Conceivably that could make for a stronger chamber depending on how the assembly is done, not unlike how the breech and barrel assemblies of artillery pieces are manufactured.
Just a guess on my part.