@whaley

If you're changing the type from non-inventory to inventory and have Initial quantity on hand, you need to enter the cost field. This is for entering inventory valuation purpose in QBO. If initial inventory quantity is zero, then it makes no difference in valuation. But this cost field data is also used for Vendor Purchase Order form. There may be other useful instances as well. Therefore, I'd suggest not to leave inventory cost field blank even if it's an estimated cost.