Limiting Factors

From the study kit.

P is considering whether to continue making a component or to buy it from an outside supplier. It uses 12,000 of the components each year.

The internal manufacturing cost comprises: $/Unit

Direct Materials 3.00
Direct Labour 4.00
Variable Overhead 1.00
Specific Fixed Cost 2.50
Other fixed costs 2.00
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Total 12.50
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If the direct labour were not used to manufacture the component, it would be used to increase to the production of another item for which there is unlimited demand. The other item has a contribution of $10 per unit but requires $8 of labour per unit.

What is the maximum price per component, at which buying is preferable to internal manufacture?

Answer: D

Direct Material $3
Direct Labour (W1) $9
Variable Overhead $1
Specific Fixed cost $2.50
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Total 15.50
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(W1) Relevant Cost= Contribution Forgone+ Direct labour= $10/2+4=9

I am trying to understand the method or logic behind the labour calculation. The contribution is divided by two. Is this because it takes two labour hours or is it related to other fixed costs in the question? Or why are the other fixed costs not used in the calculation for the answer?

Thanks in advance for any help.