Hi John
Adding to the views of Rajneesh...
1. Target cost is the cost that calculated based on actual qty delivered multiplied by plan cost and it requires for variance calculation and will update if variance key is defined.
2. Agree with Rajneesh...only the plan cost is the cost of plan qty or order qty at plan cost/price based on BOM & Routing. If you change your plan qty/order qty the plan cost will change but cost will not change on changing of BOM components and Routing once you have released the order as said by Rajneesh