A single, non-chromed, barrel just gets reamed and isn't a big deal. $65 at Epp's. Local smithies shoud be close to that.
Plan "B' is a changeable choke, like a Remchoke. $100 plus the cost of the choke tubes at Epp's. Tubes run around $10 each. Costs more because there's more machining time required. Does give you all the chokes though.