What you charge totally depends on a few things like:
1) The effort you put in the article
2) The amount of useful information you have put in the condensed form
3) How search optimized is your article for the main KW
4) Your level of grammar
5) Lastly, what rank your past articles have achieved on google search. This actually matters because over a period your client may make over $500-$5000 from a single article you wrote for the him/her for just $3-$5/500 words...
I recently did a survey of my own ghost written article and God, I am grateful that I did this. I found that many of my articles are on the first and second page of google for the targeted KW. My clients are raking in a moolah, while they are still paying me just $1/100 words. So, now I have raised my prices for normal SEO article writing to $1.5/100 words = $7.5/500 words.
In the coming future (3-4 months), I aim to raise my prices again.
Hope this helps.