On the Dashboard page in your admin, under "General Stats", there is a button for "Inventory". If you click this, it will give you a general breakdown of your inventory numbers.
Here are the line items in that report and what they mean:
"SKUs in System" - How many variants are on your database. Each condition counts separately. For example, if I have had NM-Mint & Light Play in stock of a particular card, it would count for 2 toward this number.
"SKUs out of Stock" - How many variants have a qty of 0.
"Total Value of Inventory by Retail" - Multiplies your quantities in stock by the products' respective sell prices. The sum of those is the total value by retail.
"Total Value of Inventory by Cost" - Multiplies your quantities in stock by the products' respective buy prices. The sum of those is the total value by retail.
"Categorical Breakdown of Inventory by Cost" - This is the same as the total value of inventory by cost, but it is broken down by each category.