Roofing is usually priced in "squares," which is a unit of 100 square feet. Composition shingles (asphalt & fiberglass) are cheapest in the short term, but metal panels are more cost effective in the long term since they last more than twice as long. Wood shingles, shakes, concrete tiles, clay tiles, slate tiles, and fiber cement tiles are also available, and other new products are coming out all the time.
To roughly answer your question, though, I'll assume you're a homeowner wanting a composition roof like most people. You'll end up paying about $300 per square on the East Coast, $200 per square in the Midwest, and $240 per square on the West Coast. That's the rough cost to tear off and replace a single layer of 30 year laminate/architectural shingles, labor and materials included. Metal flashing, drip edge, multiple layers, and height and steepness of the roof can all increase those costs. The current cost of oil can also increase shingle material costs since they are a petroleum based product. If you need more accurate pricing on roofing, siding, windows, or anything else, visit http://www.Assistimate.com and order a "Going Rate Check" report using their cost database.