Lots of unknowns and this isn't something anyone can give a simple answer to, because there are a wide variety of metal roof types, installation methods, and the complexity of your roof will affect the price. I will tell you that you can easily expect it to be double the cost of a complete tear off and re-roof using good shingles. My last shingle roof on 1,400 SF house was close to $10,000 just two years ago.