Material Costs can be listed in the Items section of any given Project. First, choose an Item type that has "Track Quantity" enabled. This setting can be configured from the Settings>Item Types menu for any Item Type:

When listing an item with this setting enabled, you are then able to associate Material Costs to that item using the available financial fields.

The Qty/Hours field allows you to list a quantity associated to this item, along with the type of Unit:

You can then associate a cost to that in the "Rate $" field, which allows you to set a cost for both Unit Price $ (In) and Unit Price $ (Out)

The Discount $ field can be used to express a discount in either percentage or fixed value forms.

The Tax % field is linked up to your Accounting Integration and is able to pull Tax Codes and values to be used against individual Items.

Total $ is a field that expresses the expected net revenue of a given material, your "before cost" total. This means it uses Qty against your Unit Price $ Out Rate.

Spent $ is looking at the Cost portion of the Total $ field that has been incurred thus far.