At any point in time you can see a full list of inventory in stock and the associated cost of each item by using Inventory > Report > Valuation > Valuation detailed (by item/warehouse) (report ID SA3548). This report has many filters to narrow down your results, and shows columns indicating your current average cost, quantity and other key information.
If no filters are applied, the total value shown on this report represents the total value of inventory in your system.