Is $15,000 to add R12 roof insulation during a roof replacement a fair price?

I bought my house about two years ago. It has a metal roof and no attic. I'm not sure how or if the roof is insulated or if it is to code, but I suspect the insulation is poor. The upstairs of my house is blazing hot in summer and freezing cold in winter. I am looking to replace the metal roof with shingles and have a quote from a roofer I like. I'm questioning whether adding R12 insulation for an additional $15,000 would make enough of a difference to be worth the added expense. I'm in Zone 6, and I believe code here calls for R49. I definitely do not have adequate supply to the primary bedroom/bath/dressing room that takes up a significant portion of the upper floor, and there are no dampers. I'm not sure I have the appetite for the extensive work that would be necessary to replace the existing ductwork, so I'm hoping addressing the roof might alleviate at least some of the issue. I can’t see the metal through the ceiling (which is not drywall, but decorative wood planks), but as far as I can tell there is no appreciable space between the ceilings on the upper floor and the roof.