Hi, I'm exporting data to analyze in BI applications. However, the actual cost of the product at the time it was sold was not correct, the cost changes every time there is a new purchase cost. Like I've bought an item at 100 in Jan, and sold it in Jan, in Feb, I've bought it at 90 and sold it. When exported, the cost in Jan stamped is 90, and Feb is 90. 

The filed that I'm exporting is from  "Invoice Lines/Product/Cost" which seemed to changed when there is a new cost.

Anyone can guide me where to find this cost?

I've found it from the "Journal Entry" but still need to manually calculate outside. 

The data that I want to be export are, Date, Invoice reference, Customer, Sales Price, Unit cost

Thank you very much,