Studies by the U.S. Department of Education prove that your effort pays off.
In 2007, the Census Bureau reported the following statistics:
$19,405 Less than a high school diploma
$26,894 High school diploma
$32,874 Some college or associate's degree
$46,805 Bachelor's degree
$62,287 Graduate or professional degree
The DOE has a similar table. It also generalizes, reporting that the "pattern of higher earnings corresponding with higher levels of educational attainment was consistent for each year shown between 1995 and 2007." The pattern holds for male, female, white, black, Hispanic, and Asian subgroups, they say.
If you have any doubt that going back to school is in your own best interest, study these tables from reliable sources. Research their websites. The type of job you choose, of course, has an impact on your total earnings, but a degree will land you a better job than not every time.