Fats such as butter and oils become rancid as they oxidize or "go bad" over time. Rancid oils or foods have a nasty, pugnant, acrid taste that lingers in your mouth.
Cost of what?To minimize the cost of "X" in general, you look at all the costs attendant in acquiring or making X.Then you make sure that all those costs are as cheap as they can be. If there are...