Starting with a pre-contour barrel is cheaper. Your gunsmith will gladly charge you per hour to put the taper you desire. By starting with a pre-contour that is one size larger than what is desired, he can turn it and ensure that the exterior is more centered to the bore (than before). General rate is $75 hour. The barrel should be blued to match the receiver, so the receiver may need to be done too.
If you want the contour to match the existing forearm, that will save you some labor. If he has to open up the forearm or make a new one, that'll add onto the cost.
Most fun way to do it? Take three weeks of NRA Summer School (machine 1 & machine 2) and then one week of metal bluing. At Trinidad, week one will teach you how to use the lathe. Week two you turn your barrel, head space it and fit it. Week three you blue everything.