Most adults in relatively free economic countries have a sixth sense about the value of money. The reason is less magical than it may at first appear. The closer a person is involved with the "making" of money (i.e.: generating value in trade for capital), the more they "feel" the worth of a unit of currency.
For example, if a person makes $10 an hour performing some skilled labor, then he understands $1 as being worth the strain of 6 minutes of work. Therefore, when a person is deciding to buy or not to buy, they are subconsciously considering whether it is worth the equivalent labor.
In this way, the value of goods is commonly understood throughout the world (despite varying currency rates). This is very important, but simply works by the same rules as conscious economic choices—insofar as no one is trying to abuse the customer's ignorance. However, value escapes its material bonds and grows exponentially when it is transfered into an act of charity.
When a person feels the strain each dollar represents, he naturally desires to keep it to himself. When he denies himself this gratification and instead sacrifices the dollar(s) to another person, then something magical happens. He hands over the money and transfers its value along with it, however, when he expects nothing in return, he is left with a duplicate of its value in his soul.
Furthermore, he is seen by others as a giver and is marked by value equivalent to the gift. This has a multiplying effect, because each person who appreciates the giving of the one gift attaches this value to the giver. Therefore, one $10 gift appreciated by ten people who heard about it makes the giver feel like he gave a $100 gift. So whatever high feeling the initial gift gave the giver, the spread of the story multiplies.
To top it off, when the receiver of a gift appreciates the gift, he is likely to return the favor whenever he can (in whatever form). The emotional impression made on him is likely to prompt a disproportionately large return. This, in turn, constitutes a gift—and the cycle compounds.
As abstract as this might sound, I have a real-life example. Several years ago, I was leaving my apartment on a mission to buy a pint of ice cream—about $4. Because it was a house divided into four apartments, there was only one other unit on the top floor besides mine. As I prepared to decent the stairs, I noticed a $10 bill lying on the floor.
I'll admit, my first thought was "free ice cream!" but I recovered myself. It didn't belong to me, so it must have belonged to my neighbor. I simply slipped it part way under the door, and went on my way—feeling pleased with myself.
To make a long story short, my neighbor found out that I had put it there, though I never learned for certain that it had belonged to her. Nevertheless, the dynamic between us noticeably shifted—particularly because she hadn't had much apparent experience with good-willed people in her life.
It is difficult to put a price on such "spiritual" qualities as good-will, but if one can equate money to the strain of acquiring it, then the ease that came over our neighborly contact was worth more than $1000. And really, it cost me nothing.