@IamjuViel  @Anonymous

First, QB does NOT use weighted average, it uses an arithmetical mean average.  A weighted average is like when  you were in school and tests were so much of the final grade, quizzes so much and home work so much - THAT is a weighted average.

QB uses total / qty = average, simple average or arithmetical mean average

@RKN

I do not recommend using non inventory or service type items in an assembly.

The way it work when you do.
1. you pay for the outside costs using an expense account
2. that same expense account is listed on the item
3. you do the math and calculate how much of that total cost is part of ONE assembly item and enter that amount in the cost block of the item
4.  When you build the assembly, the function will TAKE that cost out of the expense account and add it to the assembly item cost.

The pitfalls of using a non inventory or service item in an assembly
You must do the calculation in step 3 each time you pay for outside processing again - and if you did not build enough assembly item with the old cost, that is a whole new problem since cost is still in the expense account

If you set the cost on the item screen too high - building the assembly item will decrease the balance in the expense account more than it should, eventually driving the balance negative - a negative expense is additional income.