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This answer ignores the $2.00 of unavoidable fixed overhead cost.
The differential (incremental) costs are those costs that differ between the two options. The net cost of purchasing the component and renting the warehouse space is $404,000 [($37.00 × 12,000) ? $40,000]. The total cost to manufacture is $384,000 [($30.00 variable cost + $2.00 avoidable fixed cost) × 12,000 units]. The net cost differential is $20,000 ($404,000 ? $384,000).
This answer is incorrect because it is simply taking the cost differential between the full manufacturing cost and the cost to purchase, ignoring the contribution from rent of warehouse and unavoidable fixed overhead cost.
The answer of $8,000 is assuming that the $3.00 of fixed overhead that is currently applied to each component is avoidable.