This is a great question, but an honest answer isn’t simple.
Determining the cost of your roof includes many different factors. How many layers, what is the pitch of the roof, what is the condtion of the decking and what is the square footage of the roof?
Those are the basics, but you shouldn’t limit your buying decision based entirely on those factors or you will do so at your own peril.
You should investigate what kind of underlayments are used. Is it just the old fashioned tar paper? Does your contractor use an ice and water barrier? If so, do they put on more than the bare minimum of 3 feet which is required by code in many areas? And if they do, do they overlap it and continue another 3 feet? How much is the overlap? How will the chimney be flashed or will it even be flashed at all? What kind of exhaust ventilation will they provide and how did they come to the decision of how much venting your attic needs?
There is a tendency to treat roofing as though it were a commodity but unlike something as simple as sweet corn (where all things are equal), you should not simply decide on one contractors proposal for your roof against the others based upon price. If you choose that path, the old saying of “you get what you pay for” could be a hard and expensive lesson to learn.