We have done a research regarding the importance of education and how it affects the earnings among people
with and without a college degree, aged 16-65 from Bulgaria.
The income and salary of those with an undergraduate degree versus those with a graduate degree can vary widely by discipline. However, an individual with a graduate degree will generally earn more throughout his career than someone with an undergraduate degree.
Bachelors:

Question: Please tell us your average personal monthly income.
Audience: Bachelor’s, Men and women 16-65 y. o.
People who are not bachelors, masters, PhDs and don’t have interest in getting a degree:

Question: Please tell us your average personal monthly income.
Audience: Master’s, Men and women, 16-65 y. o.
Ph.D.s:

Question: Please tell us your average personal monthly income.
Audience: Ph.D.’s, Men and women, 16-65 y. o.