Yikes, Laura. Looks like there aren't many copywriters here to help, nor people who hire them. I sometimes hire an editor, but do my own writing. I recently asked my current editor (also a writer), what kind of monthly retainer she'd want to edit my weekly ezine, and she told me something that might help you as a starting point:
She said that for editing one client's weekly newsletter she charges $150/month. But for another client who sends her notes every week, and she writes the articles, she charges $200/month. That would be $50 per issue. Probably just one main article at that price. I think she's underpricing her work, considering her level of quality and decades of experience, but she's just left a university career and is new to freelancing. The example she gave me is an ongoing gig, so I can understand a lower price than for a 1-time job, and it sounds like the work is a piece of cake for her.
I did google "copywriter forum" to see what I could find out, and this is the first one that came up:
http://www.warriorforum.com/copywriting-forum/
There seems to be no "search" function on the site, to research your topic, but you might join the forum and ask the pro's what they'd advise you about getting started with your pricing.
Whatever you charge, if this is your first freelance job, it will get you started; then the path will "light up" from there. Go for it!
Suerte!