Size
Roofs are priced by the square, which is a hundred square feet of roof surface, not floor area. A single-story house has a larger roof than its footprint suggests because of the overhangs and the pitch. A two-story house of the same floor area has roughly half the roof.
Measuring from the ground or from satellite imagery gets close; walking it gets it right.
Pitch and complexity
Steep roofs take longer, need more safety gear and waste more material at the cuts. Roofs with many hips, valleys, dormers and skylights take longer still, and every one of those transitions needs flashing. A plain gable roof is the cheapest shape to cover; a cut-up tract roof with four gables and a turret is not.
Two roofs of the same size can differ a lot on this alone.
Material
Three-tab shingle is the floor. Architectural shingle costs more, luxury shingle more again. Concrete tile, clay tile and metal each step up from there, with the installation labor rising alongside the material. A tile re-lay reusing your own tiles sits in the middle because the material is already paid for.
Cool-roof requirements under the state energy code can nudge the product choice.
What is under the old roof
Removing one layer costs less than removing two. Soft or delaminated decking has to be replaced and is priced per sheet, and nobody knows how many sheets until the tear-off. Skip sheathing under an old shake roof often needs a full plywood overlay before shingle can go on.
A good estimate states the per-sheet price for deck repairs so there is no argument later.
Access, permits and the rest
A house on a slope with no room for a dumpster or a truck costs more to service. Permits and inspections are a fixed cost that varies by city. Disposal fees depend on weight, and tile weighs a lot. Gutter removal and replacement, satellite dish and solar panel handling, and ridge ventilation are line items too.
Ask for all of them on the estimate; a low headline number with the extras missing is not a low price.
Getting a fair number
Get at least two written estimates that itemise all of the above, make sure each is for the same material and scope, and ask each contractor whether they went into the attic. The cheapest number is usually the one with something missing; the most useful number is the one that explains itself.