Hi,

The code for original sale report is in the path addons/point_of_sale/views/report_saledetails.xml. You can inherit the template "report_saledetails" to add the cost price.
To know about inheriting existing report you can refer the blog below

https://www.cybrosys.com/blog/modifying-existing-qweb-reports-in-odoo13

If you need to add from Odoo itself follow the steps below

->Activate developer mode from settings

->Go to Technical->User Interface->Views->report_saledetails then edit the view like below

Regards