Quite clear, your suggestion is this, that a new feature be added to Inventory Items where a tick box “Average Cost” is added next to “Purchase Price” and when ticked the “Purchase Price” would be auto filled with the Item’s latest average cost. In addition, when ticked a percentage field would appear allowing users to nominate a percentage markup, this markup when applied to the “Average Cost - Purchase Price” would calculate the “Sales Price”.
This is similar in concept to an existing topic under the Ideas category - Cost Price Sale Price Markup Field Idea. So I will combine your excellent illustrations to that topic.