Update Items prices automatically

Are you entering the unit price, it should update the item cost. If not tick "bring back all one time messages" (general preference) enter a bill and change the unit price, you will then be asked if you wish to update the item with the new cost. I'd select do not display this message in future and select yes. from time to time you may well be offered a one off special price if this is the case only enter the extended price the unit cost will remain unchanged.