Salary
Three words: Vow of poverty. That's all you need to know.
Oh, you still need to know more?*
Sounds fun (source).
While many of the "of the cloth" folks work day and night at their convent or monastery, nuns and monks are totally allowed to work in any profession. Many choose to live second lives as social workers, accountants, teachers, nurses, and doctors.
Because of the whole vow of poverty thing, though, the nuns and monks don't actually get to keep whatever they earn. Their salaries go straight to their religious order.
In return, the order often gives each nun or monk a small living stipend. It's kind of like receiving an allowance from your parents, only you're working forty hours a week for it and you're stuck living in their basement for the rest of your life.
*P.S. That was a trick question. If you're serious about becoming a nun or monk, the correct answer is, "No, I don't need to know anything more. I am secure in a life of poverty, and money is irrelevant to my desires and path in life."
If you kept reading, well, you failed our nun/monk test. Big time.