QB Issue Resolution:

in the assembly item screen, the cost block per item listed comes from the cost block on the item screen – so check the cost block on those items that show zero in the BOM listing

However, that cost shown is a best guess kind of thing for reference only.  Cost of the assembly item is determined when you issue the build command.  At that time QB adds up the current average cost of each item in the BOM list and that total becomes the average cost for that build.

Average cost and the item cost block may or may not be the same.

Resolution for Issue 'Average cost of inventory part is blank while there is average cost, thus BOM cost is not accurate because cost of those items is zero' available: Yes (Solved).
Source: Intuit Community forum.

Average cost of inventory part is blank while there is average cost, thus BOM cost is not accurate because cost of those items is zero: this issue or error code is a known issue in Quickbooks Online (QBO) and/or Quickbooks. Support for this issue is available either by self-service or paid support options. Experts are available to resolve your Quickbooks issue to ensure minimal downtime and continue running your business. First try to resolve the issue yourself by looking for a resolution described below. If it is a complex issue or you are unable to solve the issue, you may contact us by clicking here or by using other support options.

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