You can now set your Purchase Order Unit Cost to reflect the cost of your choice. Based on the hierarchy you chose, your PO will always reflect either the item's Default Cost, the Vendor Default Cost, or the Item's Last Received Cost:

First, navigate to your Settings and then find the Purchasing tab at the bottom of the left hand list of options.
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From there, you can click on and drag the options to arrange them in the order you would like to use for populating the unit costs in your purchase orders.

Item Default Cost will use the default cost saved in your item record as the unit cost on each Purchase Order

Item Last Received Cost will use the last received cost of the item as the unit cost on your Purchase Order.

Vendor Default Cost will use the default cost listed in the vendor tab of each item as the unit cost on your Purchase Order

When you are satisfied with the order, press save.