Another very important concept used to understand mental accounting is that of modified utility function. There are two values attached to any transaction - acquisition value and transaction value. "Acquisition value" is the money that one is ready to part with for physically acquiring some good. "Transaction value" is the value one attaches to having a good deal. If the price that one is paying is equal to the mental reference price for the good, the transaction value is zero. If the price is lower than the reference price, the transaction utility is positive. Total utility received from a transaction, then, is the sum of acquisition utility and transaction utility.