A full-time school can't afford to have a lot of "part time students" who drop in to brush up on their skills a few times a year for $20. They have rent, utilities, taxes, liability insurance, payroll, etc, that they have to pay no matter what, and they need students who are going to sign up as a regular student & pay them reliably every month, on time, and come in every week for months or years.
Also: no matter what belt a student has, they still require instructor attention, curriculum, lesson planning, mat space, heat/AC, liability coverage, etc. That all has value, and that value doesn't decrease just because the person has a black belt.