Hi Arghyajit,
Your issue is still not clear. How can you compare the output of SAP with a legacy system.
First you need to identify the materials with differential values. If your legacy system values are correct and you want to maintain the same for SAP materials, you may use MR21 or MR22 as the case may be.
Once the valuation is matched, subsequent valuation depends upon the procurement conditions / material price maintained in both SAP and legacy systems for e.g. setoff / non setoff conditions in PO.