In order to find the differential cost we shall ignore fixed cost as, these costs are going to be incurred irrespective of whether the product is going to be purchased or manufactured.
Cost of Manufacture Cost of Purchase
Per Unit 11 15
30000 Units $330000 $450000
Thus as can be observed the differential cost of Purchasing the product is $120000. The company spend $120000 if the purchases the product from the third party, hence its advisable if the company manufactures it in-house.