Over ten years (1993-2003), this Single Market had boosted the EU's GDP by 877 billion euros ($985 billion)
Had brought down trade barriers which relieved taxes and other difficulties on trade
According to the EU "Delivered half a century of peace, stability and prosperity, helped raise living standards, and launched a single European currency, the euro" (EU).
Also, it "Continues to develop towards its full potential" (EU).
The money taken from each member is directly proportional to the wealth of the country
Some members such as the UK and Greece don't believe they are benefitting as much as the others.
These countries are very close to withdrawal, especially Greece who is currently refusing to pay back huge debts to the IMF (International monetary Fund).