As I said in another response, in 2005, Prime Minister Paul Martin set the following condition: Canada would become a member of that system only if no costs were imposed on it. If that condition was maintained, there would practically be no costs for Canada.
We should keep in mind that, through NORAD, Canada is already closely involved in the use of the system, at every level. A Canadian soldier is even in a room with an American soldier, and they are operating the system together.