difference between present value and updated value

Question

i have  table booingorder

obd_iTransId

obd_vCurrency

obd_vPDSNo

obd_vspareorProduct

obd_iSapareorProductID

obd_vCustItemCode

obd_iQty

obd_nUnitPrice

obd_nDiscountRate

obd_nDiscountPer

i have  data in that table will go to update price field so i need the difference at one field for past value and updated value