One spring or two
This is the largest single factor. Most double doors run two torsion springs. When one breaks, the other has the same age and the same cycle count, so a technician quoting the pair is not upselling you: they are stopping you paying a second call out fee inside a year. A quote for one spring on a two spring door should come with an explanation, not just a lower number.
Cycle rating
Springs are sold by how many open and close cycles they are built for, and the price scales with that rating. A standard spring is around 10,000 cycles. High cycle springs run 20,000, 30,000 or more. For a household opening the door twice a day, a standard spring is roughly a seven to ten year part. For a family cycling it six times a day, that same spring is a four year part, and paying more for a higher cycle rating is straightforwardly cheaper over time.
Door weight and spring specification
Springs are matched to the weight of the specific door. An insulated double door is meaningfully heavier than an uninsulated one of the same size, and it needs a different wire size, inside diameter and length. This is why a phone quote given before anyone has seen the door is a guess. It is also why a door fitted with whatever was on the truck will feel wrong and wear out its opener.
Torsion or extension
Torsion systems mount on a bar above the opening. Extension systems run along the horizontal tracks and are more common on older Las Vegas homes, particularly 1980s and 1990s builds in areas like Spring Valley and Sunrise Manor. They are different parts and different labor. An extension spring job should also include the safety containment cable, and on older doors that cable is often missing entirely.
What else the technician finds
A spring rarely fails alone. Cables of the same age are often frayed, the drums may be scored, and rollers may be worn flat. A quote that rises on the driveway is not automatically dishonest, but it should come with the old part in someone hand and a plain explanation of why it needs doing now rather than next year.
Questions worth asking before you agree
Is this one spring or both. What cycle rating am I getting. Is the spring specified to my door weight or is it a standard part. What is the warranty on the part and on the labor. Is anything else being replaced, and can you show me why. Any competent company answers those five without hesitating.