"Idioms" are frequently used phrases that have become a common way to express a thought. We use idioms all the time. There are dozens of ways to say something is expensive. The simplest is to say "it is expensive." A very common way to say it is "it costs an arm and a leg." Why? Why not "it costs a hand and a foot" or "it costs a horse and a cow?" There's no logical answer. It's just an idiom. Another way of saying something is expensive is to say "they make you pay through the nose for that." Why? It's just an idiom. A proverb or saying is usually a complete sentence. It express a whole thought, normally a piece of good advice or wisdom. An example is "Don't look a gift horse in the mouth." It means don't criticize the quality of a gift. "A Rolex watch costs an arm and a leg" is not a proverb. It's not a piece of advice or wisdom. It's just a way to say that a brand of watch is expensive. On the other hand, "You get what you pay for" is a proverb. It means that usually cheap things are not very good, and you have to be prepared to spend money to get something of high quality.

thanks dan for explaining it in detail.