The Coda market is tiny. Only a few templates will be sold. These few buyers have to compensate for the lack of volume. Nevertheless it is cheaper than asking an expert building this for you. Although the solution looks simple, it is not, but I promise you it is elegant.
Over time this template will become more expensive (that is by the way true for all templates). Make your own calculation and accept that sometimes a manuel process is the best practice. Once you have a student picking up the phone to check dates, this one may be of interest for it limits the room to make mistakes.
The only permits for selecting available time slots.