QB Issue Resolution:

cost is what you pay, so you will not get a cost unless you have entered a purchase for the item, the cost you see on the item screen is for reference

as said, check mark the non inventory item as being used in an assembly or for resale, select the expense and income accounts

use that item when you purchase it, and when you sell it. Then an item profitability report will show both cost and sales prices

Resolution for Issue 'where do I put a cost for non-inventory parts?' available: Yes (Solved).
Source: Intuit Community forum.

where do I put a cost for non-inventory parts?: 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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