Robert Corporation Has the Following Information for October, November, and December

Robert Corporation has the following information for October, November, and December of the current year:
$$\begin{array} { l r r r } & \underline{\text { October }} & \underline{\text { November }} & \underline{\text { December }} \\\text { Units produced } & 5,000 & 5,000 & 5,000 \\\text { Units sold } & 3,000 & 4,500 & 6,000\end{array}$$ Production costs per unit (based on 5,000 units) are as follows:
$$\begin{array} { l r } \text { Direct labor } & \$ 4 \\\text { Direct materials } & 6 \\\text { Fixed factory overhead } & 2 \\\text { Fixed SG\&A expenses } & 2 \\\text { Variable factory overhead } & 3 \\\text { Variable SG\&A expenses } & 4\end{array}$$ There were no beginning inventories for October, and all units were sold for $25. Costs are stable over the three months.
Calculate the difference between Robert's December income under absorption and variable costing.