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

I bought my house two years ago. It has a metal roof and no attic, and I suspect the current insulation is poor, as the upstairs is extremely hot in summer and cold in winter. I'm looking to replace the metal roof with shingles. I'm in Zone 6, and I believe the code here calls for R49. I'm considering adding R12 insulation during the roof replacement, hoping it will alleviate some of the temperature issues, especially since I don't have adequate HVAC supply to the primary bedroom/bath/dressing room upstairs and am not keen on extensive ductwork replacement. There are no dampers. I can't see the metal through the decorative wood plank ceilings, and there seems to be no appreciable space between the ceilings and the roof. I like the current ceilings and they would be expensive to replace. It seems the previous owners prioritized aesthetics over solid insulation and HVAC.