The Unit Cost field will not be updated if the quantity on hand is 0. The reason is because it's good practice to post estimated cost on the item when the item is sold even when you don't have inventory.
If you want the unit cost to be 0, do the following:
1. Positive adjust an item with quantity of 1 with 0 cost.
2. Run ACIE
3. Negative adjust the item back out
Check your Unit Cost.