The instructor supervising the solo student must remain on the ground while the student flies solo. This isn't productive time - which is why many schools charge a supervised solo rate of about $30/hr less than dual. If the instructor is a part-timer, they are paid about half their award rate for this time.
I can recall lots of hours spent sitting out in a paddock under a tree, brushing off hordes of friendly bushflies,listening out on a handheld, while a 'student' did endless circuits. Yes, it was at 'supervised solo' rates. And you think this should be free?? Wake up - the new generation of instructors won't be repeating the benevolent approach of the past.
happy days,