How much cost a graduate student to a supervisor? In order to hire a certain number of student in an office, I was wondering how much should one supervisor should think about the money involved. To restrict the answer, I'd like to focus in a biology program in a Canadian university. 

Beside the stipends (which can be very different depending on the country or the university), what is the "basic" cost of the activity in which a student would need to develop professionally. 

By basic cost, I would for example say, paying for conferences, fees of the equipment needed (laboratory accessories, computer, field equipment, office supplies, etc.), cost of publication or any activity you deem necessary to put.
As stated in the comments, this is impossible to give a general answer to; you would need to choose a field and a country. 

That said, I think the factors you list are generally pretty negligible. The major expenses are usually the stipend, tuition, and overhead on the stipend+tuition. That can easily be $60K USD per student per year, often more. Also, students require considerable training (i.e., faculty time) before they start becoming useful. Those are the main drawbacks of hiring a student; no one is too concerned about the extra pens or an occasional conference fee.