FWIW, I once converted a .45-70 Ruger #1 to a .45cal muzzleloader, in the days before somkeless muzzleloaders - so I can't exactly answer your rebarreling cost question.
When I converted mine, I D/T'd the rear of the chamber for a Remingtom 700M (muzzleloader) breechplug, to which I added a primer conversion , threading the "chamber" deep enough so that the breechplug's depth could be adjusted for reliable ingition with the primer conversion added to the plug.
I used a collapsible RR, that I carried with me, since I didn't want to add side thimbles, etc, to carry one on the rifle.
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