let's get back to square one

In enterprise, the option for user defined cost, and use BOM cost, should not even be an option, cost is what you pay, not a number you decided on.

You have stocked inventory items, you should have purchased those items to get a qty on hand and an average cost per item

then you list the items needed in the BOM of the assembly item.  When you issue the build command, those items in the BOM are removed from stock and the item average costs are added together to get the cost of the assembly item.

When you sell the assembly item, that cost is what is posted to COGS, the sum of the item average costs to make the assembly.

the BOM cost you see on the assembly item screen comes from the cost block on the item screen, it is not average cost, it is usually the most recent purchase cost.  and is just a reference value.