This is a question that is important to ask, even if it does seem a little redundant. People get estimates for all kinds of jobs, and then they become frustrated when the final cost ends up being more than the original estimate. Remember that an estimate is just that- an estimation of the costs involved. It’s not set in stone. Typically, the estimate is based on the plumber’s initial inspection. It involves what he or she believes to be wrong, as well as the materials and amount of time that will be necessary to correct the problem.
Frequently, jobs end up requiring more materials or more time once the actual process begins. That’s why it’s important for you to bring this question up before they ever start doing any work. If the person you’re considering hiring can’t provide you with any kind of an answer, it’s probably best to move on to someone that can. You should expect the final cost to vary from the estimate somewhat, but if they’re not willing to give you an idea, you could end up with a bill that is several hundred dollars more than the original estimate.