Average inventory on hand

[Jadeking]

How do I get the average cost of bats/balls we bought. I have 25 in inventory so average the last cost of 25 or whatever our inventory shows. I would need to use the date as I want the most recent average cost based on the inventory we have.
I want to get the average cost of the last 25 bats I bought. If we would only have 20 bats on inventory i would want the average of only the last 20 that we bought.