Dear,
Preliminary cost estimate is used in repetitive manufacturing area where in make to stock concept.System uses this preliminary cost estimate based on production versions.
Standard cost estimate is used to determine a cost estimate for a material at the begining of a period an dit is valid for that particular period.And we should not change the standard cost in that period so that the cost estimate is not changed and remains constant during the production setup for that period.Standard cost is not linked to production versions.
Please read the concept of standard cost estimate: Basics of SAP Standard Cost estimate- Understanding the flow of cost settings-Part 1
and Part -2 http://scn.sap.com/docs/DOC-49167
Regards,
Pardhu