Clog location- If the clog is in a difficult-to-reach position or in the main line, the cost to unclog it will be more expensive. This is usually due to the fact that hard-to-reach clogs will take a plumber longer to find/remove and a plumber may need to use additional tools depending on where the clog is located.
Main line vs. Secondary line- Your plumbing consists of several secondary lines that feed into a main line. Main lines are usually harder to access, which means if you have a clog in the main line, it will cost more to clear your drain.
Clog severity- Generally, plumbers can use a drain snake (auger) to remove a clog...but if the clog is severe enough, a plumber may have to use more powerful tools, like a water jetter, which will increase the cost to unclog your drain.
Plumbing cleanout location- Depending on the severity and/or location of the clog, a plumber may have to remove it via your home's plumbing cleanout, which is where a plumber is able to access your home's sewer line. Typically, this cleanout is located outside, in an easy-to-reach place, but sometimes it can be inside your home (inside a closet or utility room) making it more difficult to reach. The harder the cleanout is to access, the more it will cost to clear the clog.