Plumbing is a skilled trade that is heavily regulated within the State of Illinois. To become a plumber, it is not just a matter of getting a job with a plumbing company. All licensed plumbers had to do a 4-5 years apprenticeship on the job and also go to a certified plumbing school at night.
The license can be taken away by the State at anytime for continued plumbing code infractions. The differential in pricing from company to company is based upon not only overhead but also by how the plumber values his license. I have seen that those plumbers who’s pricing is well below the average pricing range do not value their license as much as those within the range.
As the saying goes “You get what you pay for.”