In my area, the material cost per square is almost identical. Metal roofers do quite well on their labor. One can actually find new number two grade metal much cheaper than shingles.
I have noticed that the price of metal really increases the further from the mills you go. My area has a lot of these mills where they make the metal from coil stock.
Metal roofs are a superior product IMO but there are some issues with insurability, installation, and fastener wearing. Also, they don't seem to do well when you change pitch or in areas where heavy snow collects or ice dams form. Also, hard to walk on when wet and roof protrusions create problems because of expansion coefficients.