What is the most expensive part of replacing a roof?
Usually the parts nobody can see until the old roof is off. Rotten decking that has to be replaced, and extra layers of old shingles that have to be stripped and hauled away, move a price more than the new shingle does.
Flashing comes next. Every chimney, skylight, wall and valley needs new metal worked in by hand, so a roof that is cut up costs more than a plain one of the same size.
Access counts too. A steep pitch needs more safety setup, and a house where the truck and dumpster cannot get close takes longer to load and clear.
To stop the hidden part becoming a surprise, the estimate carries a per-sheet price for decking, agreed before the tear-off.
Tear-off and disposal grow with every layer. Each extra layer is more material to strip, load and haul, which is one reason a roof that was covered over in the past costs more to replace now.
Ask for an itemised estimate. You will see where the money goes before you agree to anything.