@anamur-- That's why in Operations Management class, they teach us to document every transaction!
We were taught pretty much the same things about normal cost as mentioned here. The only difference is that we are asked to account for time as well. In management, normal cost is the cost of doing an activity in normal time. And normal time is the time period in which we expect the job to be completed.
It's basically the same concept, just with the inclusion of time. Time is as valuable as money in many transactions and projects. So it can also be included in normal cost.