Metal roofing is slightly more costly than a standard composition roof, namely because it takes longer to install, is made from steel, has a long warranty and, most importantly, will never need replacing. From a Portland roofer's point of view, it's perfect for this area!
The factors that make a metal roof more labor intensive, and labor costly, is the degree of difficulty associated with the job. A house that is 2 straight sides with no valleys is relatively painless, and will go rapidly. However, a roof of a lot of valleys, or is somewhat steep will make the job slower, and therefore more expensive. The material cost of steel is roughly double to that of a composition roof. Different thicknesses, or gauges, will also be influenced the price of a metal roof. Other materials that the amount of pipe affect the price of metal roofing are the pitch, the underlayment, whether or not you need to install plywood under the roof, the type of roofing felt used, the amount of pipe collars and vents added to the roof deck, and how cut up the roof is in general in terms of trying to make straight panels fit any deviations in the roof deck. Also, once your roof is off, this is a great time to add insulation above the roof deck, and below the roofing panels.
Because are so many different roofing factors, we can only tell you exactly how much is going to cost by giving you an estimate.