I have not seen this based on volume or weight, but allocation of cost by quantity should be standard if moving price is available for all articles. See http://help.sap.com/saphelp_erp2005/helpdata/en/12/08607f470311d1894a0000e8323352/frameset.htm
Also see Handling Structured Articles in ECC at the time of PO,GR and SO http://scn.sap.com/docs/DOC-29560
- Paul
Paul Gendreau
SAP Retail