When I built my house 11 years ago, I asked my builder to install a metal roof - 24 ga. colored metal, #1 grade, 20 yr guarantee. No dice. He ran backwards, claimed it would leak, etc. etc. Now 1/2 the new homes in my area are being roofed, often with lightweight 29 ga. #2 metal. It seems to hold up ok and no leaks usually. I should have fired my contractor and become a trend setter.
(actually the reason I liked them is because most houses in SW Colorado had them when I lived there.)
If the metal roof has been adequately maintained, there would be no adjustment in the Central-Eastern Ohio market. The comp shingle replacement roofs on 100 year old homes are generally there because they are about 1/2 as expensive as a new coated metal roof. And the metal roofs that were replaced only need replacement due to lax maintenance (A coat of paint every 4 - 5 years). When maintained, those old metal roofs can last indefinitely, whereas comp shingles...
If the roof has not been well maintained (showing obvious signs of rust, possibly patch work, tar around some seams, etc) and appears that it may be beyond salvage, then you should probably consider an adjustment - since the metal roof will probably need to be replaced. Maybe an effective age adjustment...
Like others have mentioned, Metal roofing systems on new housing is rapidly gaining in popularity - almost exclusively in the high-end homes - in this area.
Ray, when I grew up in south georgia we had a tin roof, and that was a wonderful sound to hear the rain on the tin roof. Even now, our carport has a tin roof and our master bedroom is near enough to here the rain on the tin roof,it is like music to my ears, my wife and I go to bed early that night.
In this area I make no adjustment for tin roofs. Wade so ga.