Depends on how well you know the machinist. If you dont know him/her very well it will probably cost you hundreds, you could probably buy a new gun for the price of having a new barrel machined.
On the other hand if you know someone and can supply most of the material it may be cost effective but you do have some safety concerns to think about. What type of blank or barrel are you starting with, is it of sufficient quality and material to withstand machining AND the pressures produced in a modern muzzleloader. There is alot of if's and but's. Custom work is usually quite pricey although the results can be spectacular if you have a machinist that is experienced with firearms or go to a gunsmith that does alot of custom work.
The key here is experience with firearms if price is not an object, making a gun barrel seems fairly straight forward for a good machinist, making a barrel that shoots well is an art. And there is still that safety concern to consider, all credible mfg's submit their barrels for independent testing and certification. They are generally designed to fail well above the maximum pressures produced by the largest powder loads that they specify. If you are not careful you could end up with an expensive hand grenade. Going forward just proceed with caution and common sense. Remember it is your face and hands at risk if something goes wrong.
