To be able to define purchase prices for the articles we will go to:

Finance → Stock Control → Products

We will see a window with the list of all the articles that we have available, we will click on the "Edit" button:

Within the tab of the article, we will go to the option "Purchase Price" from the menu on the left, and click on the "New" button in the right window, as we can see in the image provided.

We will open a pop-up window, where we will select the Supplier of that article, the reference etc ...

And we will have to put the Base Price of Purchase.