[dredmorbius]

I'd add time to that, and raw materials. Knowledge is a nonconsumable input, but education, training, and skills maintenance are also factors.

Adam Smith's Wealth of Nations has an interesting breakdown of factors contributing to wages, though he also maintains that a wage must always be sufficient to sustain not only the labourer himself (hey, this was 1776), but his wife and children, and their education so as to provide for the next generation of workers.

There are some other points I'll address directly to your parent's question.