Direct expense is an expense incurred that varies directly with changes in the volume of a cost object.
A cost object is any item for which you are measuring expenses, such as products, product lines, services, sales regions, employees, and customers. The materials used to construct a product for sale.

Examples of indirect expenses include: salaries, insurance, depreciation of equipment, equipment maintenance, facility rent , office supplies, and advertising and marketing

Total product costs can be determined by adding together the total direct materials and labor costs as well as the total manufacturing overhead costs. To determine the product cost per unit of product, divide this sum by the number of units manufactured in the period covered by those costs. The production cost per unit can be reduced by producing more with the same inputs.