You can add a costing method for each assembly products in your catalog. To do that, just go to the product you wish to edit and select from the Costing Method drop-down field.
For Assembly products, there are 4 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.
Sum Components Cost - will show the total costs of all the components in an assembly item. The cost of each components depend on the costing method selected for that specific product. You may refer to this article for more information about costing for inventory and non-inventory products: http://support.getsweet.com/en/articles/2184710-costing-for-inventory-and-non-inventory-products