To get pedantic, people get happier the more they make up to about $65,000 per year. After that it doesn't make much difference.

As to how much is enough, the common answer is 50% more... always 50% more. People compare their situation to their peers. That's why top managers feel deprived if they don't make 300 times what their lowest employee earns. They aren't comparing their status to those lowly employees, but rather to the general manager of the company in the slightly taller building across the way.