Quantifying the slope of the money/happiness curve

In this list of 10 popular misconceptions, I found some reinforcement and quantification for something I've learned over the years: having lots of money won't necessarily make you happy, but having too little money will likely make you unhappy.
"Money magazine columnist Jean Chatsky polled 1,500 people for her book You Don't Have to Be Rich and found that more money makes people significantly happier only if their family income's below $30,000, but by $50,000, money makes no difference."