Guess you are talking about SAP table VBRP?
Hm, I think this is not the right forum to ask this question, a better place would be a SAP user forum or searching the SAP help, then.
How do you define Billing Sales Margin Amount?
I could think of something using NETWR (net value) and BRTWR (gross value) fields.
But again, I think you should figure out which tables and fields to use talking to your specialist departement and / or SAP specialist / user forum.