How is the cost of the final product is calculated when I return against this final product

The BOM B is composed of Item X (qty 1) with cost 10 and Item Y (qty 2) with cost 20 USD. Currently B cost is zero

I am using average costing

Create a wip job number : J3000 for qty 3
 Complete Job

close the job

Cost relieved in Job : 150
Assembly Cost is 150

Currently Cost incurred =150
                Cost relieved =150
                Net activity = 0

X cost = 10
Y cost = 20
B cost = 50

the new cost is calculated as below: qty 1 of X = 10 + qty 2 of Y = 20 so the final cost is 50 of Item B and since the original cost of B is zero so the final cost of B is 50

now open the job