Hi RH,
To what are you specifically referring? Military musket repros? Custom rifles and fowlers? I am not aware that smoothbores are more expensive, certainly not in the case of custom guns. I do a lot of work for re-enactors and am asked to build both rifles and smoothbores. Indeed, I have 2 fowlers and a long rifle in works for re-enactors as I write, and recently built a fowler, a rifle and refurbished 3 Pedersoli Brown Besses. The smoothbored guns are no more expensive for me to make than rifles depending on the style and decoration of the gun. Smooth musket barrels are expensive but they constitute a lot of steel because of the thickness at the breech and the long length. Octagon to round fowler barrels must require quite a bit of machining, which could add cost. But again, I don't really see any big difference in cost at least with custom guns.
dave