To cost an arm and a leg:When something is very expensive then that will cost an arm and a leg. Nowaday children education are so expensive that a single parent can't afford it so minimal both father and mother have to work hard in order to meet their ends.Now a days family don't stay together because of high price of commodities due to that people are avoiding to stay in the joint family.Due to inflation each and evert person is struggling in order to get the basic things where the income is limited but the prices of each material has increased drastically increasing. 

Example - Automobile fuel and children education these days cost arm and a leg. 

A peice of cake : something that can be done very easily and for many students maths is a piece of cake and for many its is not that easy. To score good rank in IAS exam is not a peice of cake. 

Example - The English and general knowledge paper was a peice of cake. 

Let the cat out of the bag: when you accidentally tell the secret due to which you  regret later because you want to keep that secret hidden so that no one come to know about it. It happens when you are drunk and  due to which you are in unconscious state and in that ackward situation you totally get lost and in that heat of the moment you start speak your heart out. 

Example I let the cat out the bags about his marriage plans. 

To feel under the weather - When you don't feel up to the mark because you might be suffering through high fever and cold due to which you feel restlessness. I am not talking about that feel under the weather where you lie to your school teacher regarding your head ache and stomach issue.

Example - Today i am feeling under the weather because of cold and fever. 

To kill two birds with one stone -when you solve two problems just by trying to solve one problem but due to coincidence you solve both the problems.

Example - when i took elder brother to vacation then i killed two birds with one stone. 

To cut corners :when you do something so badly that it has a adverse effect later that creates a mess all around. 

Example - Contractor built the house to cut corners that their is leakage in bathroom.