You have a choice, or you could use both resources. One option is to assign both resources to each task. That way the work resource will allow you to manage demand while the material resource can be used for costing. The material resource will follow the work resource as the resource is leveled. Another option is to enter the cost directly to each task (to emulate a cost per use). Material resources have more flexibility relative to accuring the cost but either will give a cost per use/task. Hope that helps.