Stock Value is a dynamically calculated value that automatically keeps track of the cost of your stock. This is worked out from single unit cost i.e. Stock Value / Stock Level. When you acquire new stock through a purchase order the system will increase the stock value based on the value specified in the purchase order. When you sell or make any adjustments to the stock the system automatically recalculates your actual stock value based on the unit cost.
For example if you purchased 10 Mailing Bags today for $0.10 each your stock value will be $1.00, making each unit cost $0.10. So when you sold one, the stock value will be changed to $0.90. If you then purchased another 10 Mailing Bags but paid $0.15 for each, your stock value becomes $2.40 ($0.90 + 10 * $0.15) where each unit costs around $0.12, ($2.40 / 19). In other words stock value is calculated based on mean averaged value of all units.