The Crispy Baking Company is considering the expansion of its business into door-to-door delivery service. This would require an additional $13,500 in labor costs per month. Company-owned vehicles now used to make morning deliveries to restaurants could be used in the afternoons to make the home deliveries. However, it is estimated that an additional $6,000 would be required per month for gas, oil, and maintenance. It is further estimated that the home delivery use of the trucks would be allocated 40% of the existing $6,900 fixed vehicle costs. What is the differential delivery cost per month for expanding into the home delivery market