Low, medium or high price.

It’s not always so, but odds are the higher priced roofing contractor has more respect for the customer, his business and his employees, than a low priced contractor. A roofing contractor has to charge enough money  to run a successful roofing business that will be around for the long term and be able to service past customers, and to insure that the employees make a decent living.

From the Bureau of Labor Statistics:

Work environment.
Roofing work is strenuous. It involves heavy lifting, as well as climbing, bending, and kneeling. Roofers work outdoors in all types of weather, particularly when making repairs. However, they rarely work when it rains or in very cold weather as ice can be dangerous. In northern States, roofing work is generally not performed during winter months. During the summer, roofers may work overtime to complete jobs quickly, especially before forecasted rainfall.
Workers risk slips or falls from scaffolds, ladders, or roofs or burns from hot bitumen, but safety precautions, can prevent most accidents. In addition, roofs can become extremely hot during the summer, causing heat-related illnesses. In 2005, the rate of injuries for roofing contractors in construction was almost twice that of workers overall.

Some of the expenses a good roofing contractor has:
Workmans Compensation Insurance – OSHA rates roofing as the fifth dangerous occupation due to fatality statistics.This can be as much as 50% of payroll.
Business General Liability Insurance - This coverage protects you if your contractor were to damage your property or causes injury to you or others on your property.
Employee health insurance
Professional training for employees - Keeps employees up to date on new techniques and materials so the customer receives the  best job.
Professional tools and equipment - Allows workers to perform better installations in a shorter amount of time.

Material expenses.

Quality coated or hot dipped galvanized nails, or copper nails for slate,tile,copper roofing etc., as opposed to thin electroplated galvanized nails that rust and dorrode quickly when exposed to moisture.
Better formulated ice and water membranes that hold up and perform longer in demanding conditions.
Heavy duty tar paper or synthetic underlayments rather than inexpensive, or in some instances completely excluded,  shingle underlayment.