First, I wouldn't assume the same career duration for both GEDs and College Grads - generally, a dropout will start working earlier.
A general "GED average" also pretty highly weights the comparison for other reasons - poverty, area (areas with higher education and wealth levels have higher average earnings, even among those with GEDs), career type. It's an okay generalization to make in this case, but it does bias the results somewhat. People with similar levels of education/wealth tend to group together, which means a lower salary in a poorer area may have similar buying power to what seems like a higher salary in a more expensive area.
Unfortunately what might be the most telling metric is much harder to define: how much would person X make in her lifetime if she went to college, and how much would she make without? Even simplifying to the same field (their strengths would be similar in both cases), it's extremely difficult to plot her career trajectory.
Of course, this may all be due to my personal bias - college dropout, doing fine, and making well above the average starting salary for a CompSci grad by age 25. Decent savings, no debt - it was a much harder road, but I'm not sure I'd make the tradeoff knowing what I do now.
Personally, though, I can't discount the value I've felt in not having the average $27,000 in student debt hanging over my head [1]. It's enabled me to take risks, try new things, change career twice, and follow my passions.
[1] http://money.cnn.com/2012/10/18/pf/college/student-loan-debt...