You can choose from three basic types of steel door: (1) steel only; (2) steel with insulation on the inside; and (3) steel on both sides with 1-3/8 to 2 in. of insulation. Other features that add to the cost are thicker insulation and windows, especially insulated windows. The do-it-yourself tensioning systems also add a little to the door’s cost. Be sure to specify exactly what you want.
You might genuinely need some extra parts when you thought you simply needed a broken spring replaced, and a good serviceman will perform a simple inspection to identify such parts. Nor is it unreasonable for a business to charge separately for a service call versus repair work actually performed. But the best protection for you as a buyer, being somewhat at the mercy of whomever you decide to bring on site, is to understand what is being done, and ask intelligently for a clear explanation or demonstration of why extra parts are required.