There is always the option that he did not properly measure. Ask each of your contractors how many squares they think the roof is. So all is a level playing field, ask for squares before waste and accessories.
- Price should never be the sole deciding factor to hire or eliminate a contractor. - I like to tell people that quality of materials and scope of work will dictate price, but truth be told there is more to it than that.
On any given average single family residential home I will be $2,000 higher than the average roofer. But I don't consider us to be average, and though we may use the same shingle as others, a roof is a system comprised of more than just the shingles. There are the underlayments, the flashings, the ventilation. But what I sell my customers is more than just a roofing system, I sell them a roofing project. The setup and protection, the cleanup, the permit. I am also including a rock solid guarantee from myself and often bundle in upgraded manufacturer warrantys as well. So on the surface it may appear as if we are doing the same thing, but that is almost never ever the case.
You said the guy was very professional. Right there if he truly is professional he deserves to be compensated as such.
Is quality of workmanship a factor? Absolutely! I recently inspected a roof less than a year old and half the roof needs to be replaced. On paper they did everything right. In reality they did alot wrong. Valleys installed backwards to catch water instead of shed water. Nails too deep so it's like not having any nails at all, already some shingles have blown away. Chimney flashing improperly installed. ALL the other flashings were reused. Oh yeah, most roofers will reuse flashings we will NOT. The clean up was terrible, there was STILL garbage in the bushes and sharp metal scraps in the back yard.
So yeah, quality of workmanship counts. Let me tell you some other ways unscrupulous roofers may save money. They may insteall a "minimum code roof", a roof that meets the code but doesn't exceed it. Do you want the minimum on your house? They may use a quality shingle, but use cheap underlayments. I can buy generic ice shield for less than half what I pay for CertainTeed Winterguard ice shield. They may use an architectural shingle but then use field cut 3-tab shingles on the hips and ridges. They may not address your ventilation and improve if necessary. They may reuse old flashings. They might not protect your property from dust and debris generated from the roofing tear off.
There are truly soooo many variables involved in a roofing project that one would really and truly need to put all the proposals side by side and see what is being done. Then there is reputation. I charge more because I deserve more. We are all human and we all make mistakes from time to time and we've made mistakes but we fix them. I feel like a donkey when we do make a mistake, but the customers are always ultra happy we actually came back to honor our warranty.
Truth be told this is one industry where it's not hard to stand out when so many of my competitors are just plain garbage. In other words for every 10 roofers only 2 or 3 may be truly honest and reputable good people who truly care about a job well done and take pride in what they day. These 2 or 3 deserve to be paid more for their professionalism, honesty, education, training and so on.
Take a look at the link in the "sources" below to learn more about all the components involved in an asphalt shingle roof. If I assumed wrong and maybe it is not a shingle roof, take a look at the low slope options page. http://www.reliableamerican.us/services/low-slope-roofing.html