The actual cost per unit for indirect materials can be calculated using the following formula:
Actual cost per unit = (Flexible budget cost per unit x Actual activity) - Flexible budget amount
Flexible budget amount = Flexible budget cost per unit x Budgeted activity
Flexible budget cost per unit for supplies is $2.61 per unit of output, and budgeted activity for the month was 16,700 units. Therefore, the flexible budget amount for supplies was $43,647.
Actual activity for the month was 17,100 units, and the unfavorable spending variance for supplies was $6,840. This means that actual spending for supplies was $6,840 more than the flexible budget amount of $43,647, or $50,487.
Plugging these values into the formula, we get:
Actual cost per unit = ($2.61 x 17,100) - $43,647 = $44,187 - $43,647 = $540
Therefore, the actual cost per unit for indirect materials was closest to $540.