How do you adjust product cost based on quantity for accurate profit reports?

Hi all,

I drop ship some of the products on my store and my price for the product changed depending on qty ordered.

e.g

0-50 - £100

51-100 - £90

101-200 - £80

etc

I understand how to do price breaks on product price - but is there a way to do this on the cost so that the profit reports are correct? Otherwise, they’re going to be out?