Why is the PO cost not being used in work order build when the PO is issued from the work order?

Hi All

We use work orders and in some instances issue PO's from the work order (spec ord). It was my understanding that by issuing the PO from the work order it ties the items ordered directly to that work order and it carries the cost through from that PO to the work order build. The item being tied is functioning as expected, but the cost is not...how do we correct this?

Example: A work order one row, one of item # ABC on it, we issue a PO from the work order for one of item # ABC @ $100...the PO is received @ $100. A few weeks later we build the work order and the assembly build value is $50 because the average cost of item # ABC is $50 on that day. In this example we need the assembly build value to be $100 because we know that ABC was ordered specific for that work order and it cost $100 as shown by the PO being ordered from it.