Hi msp ,
Thanks for your post.
For your words, I understand that you are talking about the edit product page. Am I right?
The purchase price is a field you can manually set and we never calculate (in ATUM free). We allow updating it from the Purchase Orders setting it to the price of the PO you're editing at the moment:

However, it isn't updated on the product fields if you don't hit the button.
If you want the field to store an 'average' value, you can calculate and set it from the edit product's page.

The only way to store 'distinct' prices per product is by using ATUM Multi-Inventory. With it, you can set several inventories per product with distinct prices. You can create an inventory with its prices when you receive a Purchase Order and track when it's sold (for example).
Best Regards,
José Andrés