Make versus buy Kaplan book page 410. Access Inc.

The question requires decision to make in or outsource.
I dont undersdtand the answer in the answer section of the book. It says that buying in is cheaper. Whereby in the buying calculation the opportunity cost of 10 scooters, contribution lost in not calculated, it merely says that in both cased 10 scooters selling power wil be lost, and can be ignored as lost cash flow. In making in alternative 20 scooters lost power, which is opportunity cost is still calculated.

Could you please help me with this example?

Thanks.