Alta Company has been manufacturing 8,000 units of part $\mathrm{X}$ for its products. The unit cost for the part is as follows:

Direct materials ..... $\$ 3$

Direct labor ..... 8 Variable manufacturing overhead ..... 4 Fixed manufacturing overhead ..... 6 Total. ..... $\$ 21$

A supplier has offered to sell 8,000 units of part $\mathrm{X}$ to Alta for $\$ 18$ each. If the part is purchased, Alta can use its facilities to manufacture another product, which would generate a contribution margin of $\$ 4,000$. Seventy-five percent of the fixed manufacturing overhead costs are indirect and will still be incurred even if the part is purchased.

Compute the net differential cost in deciding whether to make or buy the part.