According to the Census, the median income for people with a Master's Degree is $54K (for ages 25-64), while those with only a High School diploma get $22K.
We need to improve our education situation 15 years ago, IMHO.
Edit: I'm going to edit this because people seem to think I'm suggesting a causal relationship here. I'm not. People with Master's Degrees tend to be highly trained in their field. It's not particularly surprising they tend to earn more.
If you invest the time to make yourself less replaceable, you will earn more money.