If your purchase orders can have many products you need a purchase orders line items table.
If a PO can have one and only one product then you dont need the line items table.
Where ever your product foreign key is located you need a unit price field.
Unit price field can be a simple number field with a scripted process to gets the unit price value from products and puts it into the field.
Alternatively you could define the unit price field to be an auto enter value that looks across a relationship to the products::unit price field. If this is the choice the make sure you choose to NOT update the value if it exists.
The goal is to capture and preserve the unit price.
I have a purchase order that draws unit pricing information from ‘Products’.
How is it drawing the unit price? Is it a field calculation?
This is a perfect example of when you need the value (unit price) from the related table (Product) to be copied across and then left alone, rather than eternally linked to the value in the originating table. As pointed out by coherentkris, in the purchase order (or line item) record the value needs to be acquired by either Lookup, Autoenter or script so that you get the value applicable at the time of the order, but then it should not change. If you are seeing it update on past orders you probably have it set up some other way.