You can add a costing method for each inventory and non-inventory products in your catalog. To do that, just go to the inventory or non-inventory product you wish to edit and select from the Costing Method drop-down field.
There are 3 options to choose from:
Fixed Cost - will show whatever amount you enter in the Fixed Cost Price field.
Last Purchase Cost - will show the cost of your last purchase. For example, on week 1 you purchase 100 units for $15 and on week 2 100 units for $20, it will show $20 on the Last Purchase Cost Price field.
Average Cost - will show the average cost of your purchase. For example, on week 1 you purchase 100 units for $15 and on week 2 100 units for $25, it will show $20 on the Average Cost Price field.
For costing for assembly products, you may refer to this article: http://support.getsweet.com/en/articles/2181992-costing-for-assembly-products