Initially a metal roof may cost more depending on the type of asphalt shingle you compare it to.  A steel roof will never need to be replaced in your lifetime, lasting 40-80 years (depending on metal type) when compared to asphalt roofs which last about 17-20 years in our climate. If you have hail, wind or storm damage asphalt shingles are more likely to damage when compared to a metal roof. Therefore over a life-time,  a metal roof is less expensive.