Markets allow us to operate according to our own plan. The price system–supply and demand–is the creation of a spontaneous order. It creates wealth for consumers and interference in the market brings unintended consequences, such as removing a species from an environment. Naturally, the demand curve works like this: if prices fall, then more people buy, and if prices rise, less people buy.
Self-interest is not the same as selfishness. Self-interest is doing what you are better off doing, and philanthropy functions on it. Mother Theresa felt she was better off doing what she did; she felt better doing it.
Price is what you give up to get something else. Mother Theresa felt the added benefit of working was greater than the cost of working.