Collection value is an estimate, not a fixed number. Prices move, and what a game sells for depends heavily on its condition.
Loose, complete in box and sealed copies of the same game can differ by a large amount. Always price the condition you actually own.
Asking prices are wishes. Recent sold prices tell you what buyers paid. Price tracking sites such as PriceCharting aggregate these.
Retro prices can swing quickly. Chamberla Collector has a refresh option for prices, and shows your total by console.
If you plan to insure or sell, check individual high-value games by hand.