Average Cost is calculated as the total cost of an item (the total amount you have paid to purchase the item or the cost entered when you transferred the item into stock) divided by the number of units of that item you have on hand.

Last Cost is the most recent purchase price for the item.

Standard Cost can be used instead of the last purchase price as

the default price on purchases. To use the standard cost price as the default price, select Use Standard Cost as the Default Price on Purchase Orders and Bills preference in the Preferences window (Stock tab).