Proverbs 13:11 (NIV)
Dishonest money dwindles away, but he who gathers money little by little makes it grow.
When asked "What is the greatest power you ever witnessed?" Albert Einstein answered, "compound interest!" The secret to compound interest is that you have to spend less than you earn. The Bible says that money gained by dishonest means will dwindle away. And why is that? Because the value of a thing is measured by the cost or the effort of obtaining it. Working hard for the money earned makes you appreciate it and encourages you to keep it.
We say that salvation is a free gift. And from the above argument you might think it has little value. Yes, it is offered freely to us but that does not mean it did not cost. It cost God the death of His Son, Jesus Christ. And that makes salvation the most valuable thing we could ever possess.