Maximum pricing question

Hi Sir,

I present to you another question today that I am stuck on.

Q:

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:
Direct materials – 3$/unit
Direct labour – $4$/unit
Variable o/h – $1/unit
Specific fixed cost – $2.50/unit
Other fixed costs – $2/unit
Total – $12.50/unit

If the direct labour were not used to manufacture the component, it would be used to increase the production of another item for which there is unlimited demand – this other item has a contribution of $10.00 per unit but requires $8.00 of labour per unit.

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

The answer provided:

Direct material – $3
Direct labour (W1) – $9
Variable o/h – $1
Specific fixed cost – $2.50
Total – $15.50

W1 – Relevant cost = contribution foregone + Direct labour = $10/2 +$4 = 9

I am confused about why contribution foregone is $10/2. I’m not sure where the 2 comes from. Could you please help me in explaining this.

Regards,
Yazan