At the beginning of the year, Gizmo Inc. is considering whether to repair and retain an existing machine, or to replace it with a new machine. The following information is available to analyze this decision:

Machine overhaul costs $($last year$)$ $ $6,000$

Repair costs $($current year$)3,000$

Annual operating costs $($existing machine$)14,000$

Annual operating costs $($new machine$)10,000$

Which of the costs being considered for this decision represents a sunk cost?