What It Costs
Two quotes on the same opening can differ substantially, and almost all of the difference is in the construction and the hardware rather than the appearance.
Short answer
The drivers are the door size, the construction (single skin to insulated sandwich panel), the hardware grade, the spring cycle rating, the opener, windows and decorative options, and whether the opening needs any carpentry. The cheap quote is usually a single-skin door with entry-level hardware and standard springs.
What drives it
Size: a double door is more than twice a single, because the hardware and springs scale with weight as well as area.
Construction: single skin, single skin with an insert, polystyrene sandwich, or injected polyurethane sandwich. This is the largest single variable.
Hardware grade: track gauge, hinges, rollers, brackets and bearings.
Spring cycle rating, which on a door used as the main entrance should be above standard.
The opener: drive type, motor rating, battery backup.
And windows, decorative hardware and a non-standard colour, which are real costs and are genuinely optional.
Where the cheap quote saves
A single-skin door, standard-gauge track, plain steel rollers, standard-cycle springs, an entry-level chain drive opener, and the existing seals left in place. Every one of those is a defensible choice on a detached garage nobody uses much, and every one of them is the wrong choice on an attached garage used as the main entrance with a room above.
What should be itemized
The door: model, construction type, insulation, colour and any windows.
The hardware: track gauge, roller type, hinge grade.
The springs: cycle rating, stated.
The opener: model, drive type, motor rating, battery backup, number of remotes and whether a keypad is included.
All four seals: bottom, sides, header, and between sections — plus a threshold seal if the slab needs one.
And the labour, the removal and disposal of the old door, and any carpentry to the jambs, the header or the stop moulding.
The things to spend on
Insulated sandwich-panel construction, which affects warmth, noise and rigidity together.
The hardware upgrade, which is a modest premium and changes how long the door lasts.
A higher spring cycle rating.
Nylon rollers.
A belt drive or wall-mount opener if there is a room above the garage.
And the seals, which are the cheapest part of the performance and are routinely left as-is.
Common questions
Why do garage door quotes vary so much?
Construction type, hardware grade and spring cycle rating, not appearance.
What should be itemized?
The door, the hardware, the spring cycle rating, the opener, all four seals, and the carpentry.
What is worth paying extra for?
Insulated sandwich panel, the hardware upgrade, higher-cycle springs and nylon rollers.