I have a idea: can you hide the unit and amount price for everyone, then I will set profile for each user who have permission, he can't see the unit and amount price on sales orders page, but he can click purchase order, then he can see the detail on purchase order page.
Who don't have permission, he can't see the unit and amount price on sales order page, also he can't go inside to purchase order to see detail.
If this way is not work, please just set as you said, I can accept "profile will be read only of Purchase Order function