The first thing you should do is visit a club. Take a look at the facility; is it in a shared facility? A church/ basement/ or community center? If so these programs should be available to the student at a much lower rate (as they do not pay overhead like a full-time martial arts school).
Talk with the instructor and observe how he/she is teaching. It is very important that you feel comfortable with your instructor and the way classes are being taught. You must be able to develop trust in your instructor.
Listen to yourself or your child; do you have positive thoughts and comments about the school and the instructors? The style that is taught is not really important, both ITF and WTF TaeKwonDo are, like any martial art equally good, they are just a bit different (I always say, “The martial arts are all music, some are just written on different sheets”)
Do not pay fees that are too high, a good instructor should be teaching for the love of the art, and not to become rich!