It's very rare for an o/u barrel to drop fit into another receiver. How much a gunsmith would charge depends on how much work needs to be done. Usually some steel needs to be taken off the barrel to properly fit the locking lugs and some adjustment to the barrel where it fits to the hinge pins. The steel has to be taken from the barrel as any change to the receiver means it won't fit with the original barrels. In extreme cases metal may have to be TIG welded to the receiver and then shaped to fit.
FWIW, some guns are more easy to fit than others depending on the design.