The door is not the only material cost. Interior and exterior trim work, patch materials (drywall etc), paint (interior/exterior walls/door and trim), caulk, silicone, nails, screws, sill waterproofing, possible siding, insulation, saw blades, knife blades, disposal of the old door, fuel, door hardware, keys, flashing.
The contractor had to come to the house, measure, talk with the customer about their options etc. They had to check the existing trim work and figure out where to source matching trim, hinges and door hardware. They had to figure out paint colors for interior and exterior trim. Figure out hinge colors and door hardware including locks and keys. Make a material list. Tally all the costs, go to the supply house and gather door prices. Create the bid for work. Submit the bid to the customer. Discuss and agree on a contracted price. Order and gather all proper materials.
Then they had to figure out all tools that would be needed for the job. Bring tools to site and set up. Ensure all materials are on site. Protect work area from dust, damage etc. Demo old door. Asses framing etc for level, rot, flashing or drainage issues. Waterproof sill. Address any flashing and siding issues. Install new doors. Insulate, trim, caulk, touch up any interior sheetrock or exterior siding issues, fill nail holes, sand, install door hardware. Then clean worksite, dispose of debris, return tools. Next day one or two coats of paint. Likely one more trip to the site for paint touch-up and to ensure customer is satisfied. Lots more involved than just "putting in a door"