Work Ticket By Product - What cost does it go into inventory at?

Some of my work ticket's create a by product item that goes into stock.  We have been doing a line on the work ticket materials tab for -1 Part Number to remove it from the work ticket and return it to inventory. 

How to do we control what value it pulls off of the work ticket and goes into inventory at?

It is a part that is never bought to stock so there are no goods receipt values for it to look back to.