River Valley Incorporated $($RVI$)$ currently grows the grapes used in producing its record$-$breaking wines. The company currently produces and sells $23,800$ bottles of wine per year. Due to changing weather conditions in the region, RVI is considering purchasing its grapes from an outside supplier. RVI located a supplier that is willing to grow the grapes to the same level of quality for $$16$ per bottle. Regardless of RVI$$s decision, the company will continue to be responsible for all marketing and selling of its products. RVI is currently spending $$21$ per bottle for grapes, which is made up of the following:

Item Unit Cost

Growing materials $$6$

Farm labor $5$

Indirect growing materials $1$

Advertising cost $9$

Total unit cost $$21$

Required

Calculate the avoidable cost per unit. Should RVI continue to produce its own grapes or purchase the grapes from an outside supplier?