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This answer includes the fixed manufacturing overhead in the costs that would be avoided if the parts were purchased outside. However, the fixed manufacturing overhead would continue and so it would be the same whether the parts are manufactured in-house or purchased outside.
Total per unit manufacturing cost for the T305 is $42,400. Since the overhead of $24,000 is 1/3 variable and 2/3 fixed, $16,000 of that would continue and $8,000 would be avoidable. Thus, the total avoidable variable manufacturing cost per unit is $26,400 ($2,000 DM, $16,000 DL, $400 materials handling, and $8,000 variable manufacturing OH). If Richardson Motors purchases the units from Simpson Castings then the per unit variable cost will be $36,000 [$30,000 purchase price + $6,000 material handling cost applied [20% × $30,000 per unit]. Therefore, by purchasing from Simpson Castings the per unit cost of the T305 component will increase by $9,600 ($36,000 ? $26,400).
This answer does not include the materials handling cost for the parts purchased outside.
This answer includes the fixed manufacturing overhead in the costs that would be avoided if the parts were purchased outside, and it also fails to include the materials handling cost as a part of the cost of purchasing the parts outside.