There isn't much to add to Ken Stern's article in The Atlantic magazine about the relative giving habits of individuals based on their income.
In 2011, the wealthiest Americans—those with earnings in the top 20 percent—contributed on average 1.3 percent of their income to charity. By comparison, Americans at the base of the income pyramid—those in the bottom 20 percent—donated 3.2 percent of their income.
Read the whole thing and pay special attention to the impact of exposure to need.
At Downtown Church we've conservatively estimated based on past trends in our giving that if every household gave 1.5% of their income, assuming an average household income of $60,000/year, we would have more money than we need.