Do you have a thirst for knowledge and a knack for research? Do you have an advanced degree in a particular subject? If so, academic writing may be a great niche genre for you that pays well. Those with advanced degrees can make good money writing course material, researching and compiling educational resources, tutorials, textbook materials etc.
There are of course more dubious ways to earn income with academic writing. There are websites where you can sell research papers, essays, and even dissertations as "examples" for other students to purchase. This is a polite and indirect way of advocating cheating in my opinion and if you believe in karma, I would avoid selling papers. This is of course only my personal opinion, do with it what you will, but think about it. Would you want to go to a doctor or hire a lawyer who had paid for their papers/research? I would only be inclinded to trust those who have put in the actual time and effort it takes to study and research yourself. If someone is too lazy to do their own work, they are most likely too lazy to be effective in their career. - soapbox over ;).
According to SimplyHired the average salary nationwide for an academic writer is $49,000. This of course will vary widely depending on the type of academic writing you are doing, but still that's not a bad average.