First some comments, then I'll try to answer your questions.
Just because you put in ridiculous amounts of hours developing your skills does not automatically qualify you to make good money. If this were the case, music performance majors would be the richest people in the world. If your skill is not in demand, people won't pay for it, regardless of your effort.
Demand for professional editing in graduate work is reduced for a couple of reasons. First, every graduate student has an advisor. I know my advisor is a "grammar nazi" and in any case it is in their best interest to polish papers, of which they are often co-authors, through whatever means they can because their professional reputation is on the line. And if the paper is a thesis or dissertation, it will have to be read by additional members of the faculty. Second, editing is not heart surgery, and lots of people are good enough
and willing to do it for free. Despite being in a technical field, I know an awful lot about grammar and editing and have read over many a fellow student's paper. I may miss something you wouldn't, but chances are the error is benign enough to take away nothing from the paper. Third, there are other official university resources. I don't think they offer straight-up, free editing, but there are ways of getting help with writing.
So keeping those things in mind, how do you advertise and how much should you charge? To advertise you have to put your name out where these people are likely to look. Do you think non-native English speakers are going to read the Daily Universe classifieds? Or peruse the Wilk Board? Then advertise there. I think your best bet is to advertise where the students actually are. See if you can post flyers in the graduate areas of buildings with the most non-native English speakers (I would guess the technical buildings, like Clyde, Talmage, Eyring, etc). See if you can leave flyers at the Writing Center
, or any other university writing resource. And don't forget timing. Ask around to see if there is a time when students are more likely to be writing papers (end of semester?), so your ads are more effective. Think like a grad student. Where would you look?
Pricing is the tricky part, as you know. Honestly, I just don't see a demand for pricy editing, so it's going to be a cost-benefit decision for you. What I can tell you is that as a grad student, I would be uncomfortable with an hourly rate, not knowing how fast you work and how much I was going to pay at the end. I think my preferred payment method would be a lump sum for the paper. Perhaps with ranges, like $XX for 0 to 20 pages and $YY for 21 to 50 pages.
Oh, and one more comment, for what it's worth. Writing styles for various disciplines can be very different. I have several times made the mistake of using a word in what otherwise would have been acceptable usage, only to find out that it had a very specific meaning in my field and my usage was implying something I didn't mean. There are issues of word choice, style (technical writing is often extremely compact compared to flowery literary papers), and conventions you would have to watch out for. You're probably aware of this, but I thought I should mention it.