@GeorgeIoak 

ahhh, to get to the point of the original question

QB keeps average cost for inventory items, in premier and pro there is no other option. When you sell an inventory item the average cost of that item is posted to COGS. Regardless of whether you buy one, 50, or 2500, QB calculates a new average cost. total value / total qty = average cost.

When you enter a purchase, you enter the qty and the actual amount you paid, average cost in QBDT spreads that out across the qty in stock.

In the assembly item you specify the item and qty.
The cost block on the assembly item does nothing in terms of impacting assembly item cost when you build it.
The cost block on the item screen does nothing either if you are entering a purchase of the item as required.

IF, if you have not turned on multiple unit of measure, I do not recommend using it, see this for why
http://onsale-apparel.com/Rustler/tag/um-good-and-bad

Without multiple u/m, you enter the qty actually receive, a case of 2500 would be entered as 2500 each and not one case.