In this specific example, the meaning of the phrase is different from it's original meaning, but related. I'll explain. A pound is unit of measurement of weight in the American standard system. We americans have our own unique systems for measuring weights, distances, and volumes. It's usually called the standard system, I believe. The metric system is also used, but not as much. As a reference, college textbook of 500 pages probably weighs about six pounds. Grams, Tons, miles, liters, pints are some other units of measurement in use here. If some kind of good is sold 'by the pound', it means that a pound of that item costs a certain amount. For example, a pound of beef might cost one dollar or a pound of apples might cost 50 cents (half of a dollar). So I hope you can see, this is how the phrase originated, and how it is used in this context in a way that has a vaguely similar meaning.