Hey, RollingGreen! I'm a little late to the party, but I kinda have experience doing this in the academic realm--I was a tutor at my university's writing center while studying, and when tutors left work there, administrators offered to put them on a freelance editing referral list. From being on that list, I was eventually able to become an "approved proofreader" for the same university's School of Communication, which contains about a dozen graduate degrees in the field.
Graduating thesis- and dissertation-writers at this school are required to get their work looked over, so I get a fairly reliable stream of about 5-10 of these long papers every year (there are only 2 other approved proofreaders to my knowledge). Depending on how much I charge per page and how detailed students want the editing to be, I could net anywhere from $60 to over $500 for one paper. Academic proofreading is fairly straightforward if you have fair knowledge of the main style manuals out there (MLA, APA, Chicago/Turabian) and/or are able to check out the manuals from your local library. I haven't ventured into other realms of writing yet, and I don't know what other universities' procedures and opportunities are, but I could imagine it being pretty lucrative if you get your foot in the door for multiple departments at a mid-size university.