The Average Cost is computed by QB and READ BY AO ...AO does not manufacture the Average Cost of this item, it is read by AO from the QB database when a sync is done. Therefore the first thing that should be done is to sync the item.
** Make sure that your sync queue is empty and run a selective sync with the Items boxes checked. If your sync queue is not empty then this will not work**
Also please check the Average Cost of this item in QB...if in fact it is wrong there, enter a value adjustment in QB to correct it...having done this and showing the correct Average Cost on the QB item list then go ahead and do a Sync from within AO. This should correct the Average Cost in AO as well