Obtaining COGS Average

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Hi

I am trying to obtain an accurate stock value at the end of each month. The task is proving difficult as our COGS for each product changes throughout the month when purchased. 

The shopify backend does not provide an average cost price for the same product bought at different purchase prices throughout the month. 

I am aware the stock app may handle this issue but we do not have the POS pro subscription so we cannot download this app. 

Is there any other simple workaround or app that can help us resolve our issue.

Thanks in advance