First, ensure you have Original Cost as a Attribute on your items (Article w. video).
It'll also help if you have this Attribute on your Inventory List as a Custom Column (Article).

Then, select all your products and download the inventory to an excel document (Article).

From here, let's open the download in Google Sheets (I find this the easiest to use)
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Next, let's remove the top three rows (for ease of navigation)
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Then, add in a new column for 'TOTAL VALUE'
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After, add in a command to multiple Original Cost by Quantity Posted. =MULTIPLY(cell with original cost, cell with quantity posted)
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Apply this calculation to all cells.

Finally, sum together all those rows.
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And there you have it! your total for all items in your inventory.