Plumbers charge both by the hour as well as by the project. In many cases an hourly charge may be accompanied by a “service call” fee. This fee is for coming out to take a look at the issue.
Hourly rates vary widely across the US. On the low end, you may have a plumber that charges $40 per hour plus a $70 service fee. On the higher end, you may have a plumber charge $150 per hour with or without a service fee. A master plumber is going to be on the higher end of the pricing spectrum, while a journeyman plumber is going to be on the lower cost side of the pricing range.
An unlicensed “handyman” might offer to work on a plumbing issue for even less. But unless you know your handyman and his level of skill, you might just be saving a little now and paying more later to have the cheap fix, repaired by a more expensive pro later.
Your fee schedule must make sense for you as a professional and take into account what your particular market will bear.