You can set buying price while editing supply. Updating buying price in supply you also update it in price list.

Also you can edit buying price while editing price list item:

1. 1. Open the price list

2. 2. Press Edit in the top right corner

3. 3. Select the price list item you want to edit, the item editing window will open.

4. 4. Update field beside Buying price

Note: Changing buying price in price list item does not change it in existing supplies!