As far as cost of metal roofing without exposed fasteners, you are correct in that those types could easily be 4x the cost of a roof with fasteners showing. Also, a do-it-yourselfer can install a metal roof with exposed screw fasteners whereas the types without exposed fasteners are best left to those with the right tools and experience which usually means lots of money! The metal and fasteners for my roof (3900 square feet of metal) cost me about $2500.