Well, on the lower end, there are roof contractors underbidding their jobs because they are new to the industry and pricing low to gain experience as well as exposure. As compared to an experienced contractor, a contractor that is new may be more willing to take on projects at razor-thin margins to increase exposure and build credibility.
On the higher-end, we have fully-warrantied jobs from reputable companies that charges a high price for their services, However, being expensive does not always ensure quality. Companies might subcontract to other contractors at a low price in order to save costs and reduce workload. This means that sometimes they may have to cut corners due to the low revenue.
However, many highly trusted and professional contractors still stick to the "40% materials and  60% labor" guideline as their costs-breakdown formula. With that said, it does not mean that this pricing structure is strictly followed. It just serves as a basic guideline for better results. We have been in this industry for years and strive to produce the best results at an affordable cost. Specializing in roof installation, roof repair, roof waterproofing and roof leaking repair.