149. A supply of any service, other than a supply referred to in section 148, made by a public service body in the course of an event or activity, is exempt where the total of all amounts, each of which is the consideration for a supply of such a service made by the body in the course of that event or activity, could not reasonably be expected to exceed the aggregate of all amounts each of which is the direct cost of a supply of such a service made by the body in the course of that event or activity.