Hi Sudhakar,
You can see how this is calculated in transaction SE37 in function module MRM_AMOUNT_QUANTITY_PROPOSE.
A common calculation of the amount proposed is:
E_WRBTR = E_BPMNG * I_MRM_PROP-NETWR
/ HV_BSMNG.
where:
E_WRBTR = invoice item amount
E_BPMNG = goods receipt quantity in PO price unit
I_MRM_PROP-NETWR = net order value in PO currency
HV_BSMNG = Purchase order quantity in PO price unit
Hope this helps,
Adam.