Hi there, Steve Rutt.

Welcome to the Intuit Community. Allow me to bring clarifications to your concern about Inventory Assembly in QuickBooks Desktop.

For the items that you build, the Avg. Cost will be the total of the costs of all the items in the Bill of Materials. However, you're still responsible for filling in the Cost amount yourself. The initial Avg. Cost of the inventory assembly is calculated by dividing the Total Value with the Initial On Hand quantity.

You can either select the Use global preference which is your setup in the Items & Inventory preferences, the User defined cost which is use if you want to set the cost yourself, or the Use Total BOM Cost if you want to use the total cost of the components in the Bill of Materials as the total cost of the assembly.

In addition, QuickBooks uses the weighted average cost to determine the value of your inventory and the amount debited to COGS when you sell inventory. The average cost is the sum of the cost of all of the items in inventory divided by the number of items.

To learn more about Inventory Assembly Items in QuickBooks, you can check out this article: Create, build, and work with inventory assembly items.

Should you have further questions about building assemblies in QuickBooks, just let me know. I'm always here to help.