Transfer pricing problem

Dear Sir, could you please help me with this problem?
There was a question about setting transfer price in ACCA past exam which is as follows:
C Co is the division selling components to Gearbox – another division of the same company and C Co is currently working to full capacity. The company’s policy is that divisions must always make internal sales first. C Co currently satisfies 60% of the external demand for its components. Its variable costs represents 40% of revenue.
The financial results of Co is as follows:
– External sales: $8010
– Sales to Gearbox: $7550

Here is what i did to determine the minimum transfer price
The current external sales are 8010, so 40% more sales could be made externally is 40%*(8010/60%)= 5340 -> this is the lost sales if Co makes internal sales instead of external.
-> Opportunity cost – lost contribution: 5340 * (100%-40% var cost) =3204
Plus: variable cost of internal sales: 7550*40%=3020
So the minimum price should be set is: 6224$

However, here is the answer to the question
From C Co’s perspective, of the current internal sales of $7,550,000, $5,340,000 could be sold externally if they were not sold to the Gearbox division. Therefore, in order for C Co not to be any worse off from selling internally, these sales should be made at the current price of $5,340,000.
As regards the remaining internal sales of $2,210,000 ($7,550,000 – $5,340,000), C Co effectively has spare capacity to meet these sales. Therefore, the minimum transfer price should be the marginal cost of producing these goods. Given that variable costs represent 40% of revenue, this means that the marginal cost for these sales is $884,000. This is therefore the minimum price which C Co should charge for these sales.
In total, therefore, C Co will want to charge at least $6,224,000 for its sales to the Gearbox division

Though I gave the same result as the answer, I was wondering whether my way of thinking was correct. And if I was right, could you please explain to me which method is more reasonable and regularly used?

Thank you!