Just a couple thoughts.
When determining the cost per student, remember to consider all the costs of bringing that instructor. Add travel expenses if nessary - mileage etc as well as the daily or per student fee.
Be sure all know the supply requirements and if the instructor has a kit, ask if it is mandatory to buy the kit or if students can make their own. Your original post appears to indicate there was some miscommunication somewhere about the number of kits required and if there might be any extra for purchase after the class. It is a good idea to do this after the class and not before so all will have sufficient supplies.
In your supply list be sure and add a statement about machines being in good working order and all accessories needed are brought including the manual. One would be suprised how many students don't even bring the necessary special foot and wonder why other feet won't work.
If it is an all day workshop, be sure and provide food for the teacher or if other arrangements are made.
Another of your question about how a teacher charges.
Some on a set fee depending on the length of the workshop plus the travel/mileage
Some a set fee up to a certain number of students and then a per student additional after that.
Some will ask for a per student fee.
Some wil have a fee which includes kit and some not.
When making initial contact with a potential teacher, it is helpful to give as much information about the group as you can, possible class size etc.
If the initial contact is a general inquiry be sure and let the teacher know what your decision is. I am sure, as I do, pencil in the dates to avoid conflict. If you decide not to have the teacher then he or she can release the date.
My opinion of one of your comments is: If the cost per student was based on 10 students and there were more, then after the expenses for the teacher was paid then the rest would go into the treasury of the guild unless before hand there was a decision made to reimburse each student. Some guilds will give an estimate of the cost per student and make the final determination after sign up.
Above all, a guild should avoid any misunderstanding, confusion on the part of teacher/student and program director.