How much does it cost to replace a roof?

That depends on a lot of stuff. How big is the roof? What quality shingle -- assuming it's shingles -- are you going to replace it with, 20-year, 30-year, or even better? Are you going to lay the new shingles right over the old or are going to rip up the old? If you tear up the old, what is the condition of the roof under the existing shingles? Does a lot of wood need to be replaced or repaired? Are you doing the work yourself or are you hiring a contractor?