a monk's earnings are pure profit
Proverb meaning that since monks traditionally have no capital or expenses, any income is pure profit. Used figuratively for businesses or situations with minimal overhead.
They say a monk's earnings are pure profit, but in reality maintaining a temple costs money too.
This business has almost no cost of goods, so it's like a monk's pure profit.
The phrase originates from the idea that Buddhist monks traditionally had no capital investment or living expenses, so any donation or income was entirely profit. The exact historical derivation is uncertain.