I’ve been ghostwriting for over 3 years now. Some of my work cannot be made public so it’s difficult to make an online portfolio; rather, I just have freelancer profiles and testimonials. I’m charging around $80-$100 per 700 words on average (depending on type of content). Usually more for larger projects.
I know this goes without saying, but you should at least be charging enough to “live” if this your main source of income. That said, I’ve seen ghostwriters charge $200 – $400 per article on high authority websites. You might also consider applying to About.com sections where writers are paid a decent amount to consistently write 300-600 word pieces.