Roof replacement
Two quotes for the same roof can differ by half, and the reason is almost never the shingle. It is what happens underneath it, and what happens if the decking turns out to be soft.
What is actually being priced
Tear-off and disposal — how many existing layers come off, and where they go. Decking repair — priced per sheet, and the number of sheets is unknown until the old roof is off. Underlayment, and whether ice-and-water shield goes at the eaves and in the valleys or only where code requires it. Flashing, and whether it is replaced or re-used. Ventilation. Then the shingle itself, which is the part most homeowners compare and the part that varies least.
A quote that looks low is frequently a quote that assumes the decking is sound and the flashing can be re-used. Both assumptions get tested on day one.
Layers, and why they matter
A second layer of shingle installed over a first is legal in many jurisdictions and it is cheaper, but it hides the decking, adds weight, and traps heat that shortens the life of the new material. It also means the next replacement is a bigger tear-off. If a house is on its third roof and two layers are still up there, that is a different job from a clean single-layer tear-off.
Flashing is where roofs leak
Not the field of the shingle — the transitions. Where a roof meets a wall, a chimney, a skylight, a vent, or another roof plane at a different pitch. On houses with attached garages and porch roofs, which is most of the postwar housing across this region, those junctions are numerous.
Ask specifically whether step flashing along walls is being replaced. Re-using it saves a few hundred dollars and is the single most common reason a new roof leaks in its first year.