Thanks for keeping me updated about this, @EL34.

To verify why you're unable to access the average cost through the QBXML, I recommend reaching out to our Intuit Developer. This team is the best resource for such concern as they have the tool to check your software libraries. 

You can visit them through this site: Intuit Developer

I'm also adding here a link about using the QBXML in QuickBooks Desktop for reference: qbXML Message structure

In the event that you have any other questions with QuickBooks, let me know. I'd be happy to help. Have a great day!