as usually happens, but often we do get surprised. In my very last experience there was this article charging around $10 per article (had him for almost a year or something and he would constantly provide me 10 a months). However after few miss understandings i had to move on and find another writer.
I ended up finding an excellent young guy, eager for knowledge and money that would provide me "detailed" reviews of the product for which I wanted the articles to be written at half the price, double the words and very well optimized for the keywords (yeah, I did paid him much more then what he asked, but all of the articles I have from him are just gaining excellent rankings and prove to provide excellent backlinks from citations etc).
But back to the standard pricing. It quiet depends really on what type of article you are after and who is it writing it. You can find a professional writer that would charge you $30-40 for a 500 words article as well as you can end up finding a very talented person that is willing to write same quality of article for much less then that ($10-$15). With most of the writers I have worked were in the $10 range for 500 words article.
Just my 2 cents from my experience...