1. There's no such thing as a free lunch.

= Nothing in life is free! One way or another, you’ll sooner or later have to pay for something that was, at first, given to you for free.

This saying seems to have been coined in response to the democratic stance of Henry Wallace, the US Vice President (1941 -1945). After World War II, he proposed providing "minimum standards of food, clothing and shelter" for people across the globe and added, "If we can afford spending tremendous sums of money to win the war, then we can afford investing whatever amount it takes to win the peace.”
His opponents were quick to argue that “one will always have to pay for a free lunch”, implying that Henry Wallace's intentions were far from charitable or unselfish.

This saying might be used by someone who has to work a lot to get what they want. 

2. Money makes the world go round. 

= Money is the only reason that the world keeps turning.

This phrase is from the world-famous musical Cabaret, written in the sixties. 

This saying might be used by someone who believes that trade or the stock market, for example, plays a vital role in everyday life.
 
3. Neither a borrower nor a lender be.

= Avoid borrowing money from other people or even lending it.​

This saying originates from Hamlet, written by William Shakespeare in 1602. In one of the acts in this play, Polonius advises his son, Laertes, before the latter sets out on a journey.

This saying might be given as a word of advice to a young person who lacks experience in managing their finances. 

Below is a list of ten phrases that are rather tricky to understand the first time you hear them or read them mainly because at first they give the impression of having a literal meaning. However, if you pay closer attention to the context, you will soon realise that they're, in fact, English idioms!

1. a wet blanket
if you call someone a wet blanket, you mean that they ruin other people's fun by being boring and miserable or by disapproving of their activities 

Example: 
- Oh gosh, I'd rather have stayed home. Why did we have to come here? It's so crowded and the music is almost deafening, I can't stand it.
- Oh, don't be such a wet blanket, Tom! Stop complaining, get yourself another drink, and try to have some fun. OK?

2. rain check
if you say you'll take a rain check on something, you mean that you cannot accept an invitation or offer now but you would like to do so at a later time

Example:
Do you mind if I take a rain check on that drink? I don't feel like going out tonight. 

3. a dark horse
a person who has interesting qualities or abilities that most people do not know about

Example:
The shy little girl turned out to be a dark horse in the dancing competition. No one knew she could dance so gracefully. 

It may also mean "a secretive person".

Example:
You're such a dark horse! Why didn't you tell me you'd got promoted?