Opportunity costIf you farm herbs, you mill them, create inks, and then use those to make armor vellum, those vellums are not free, nor are they the cost of the vendor paper. The farmed materials can be sold a certain amount, and many people neglect to factor this. Farming feels like getting something for free -- after all, you're playing a game, and then when you finish playing, you have goods you didn't have before.
Opportunity cost is, simply, what something would have been worth had you done something differently. The opportunity cost of going to law school is not the tuition, but the money you would have made had you not been studying law. Add that to the direct cost (tuition, books, bribes, what have you), and you get the total cost. Compare this to the profits at the end of the run to determine whether it's worth it.