I use getafreelancer but you have to be careful, ask to see peoples work first and then get them to write you a 300 word article on a subject even before selecting them.

Then take that article apart and try and work out if it is a copy and past job or a direct copy. Make sure the article flows, I can't write for shit and my sentences never flow and most of the time I sound like a 10 year old after 2 cans of coke. I want the article to be easy to read and flow well and be interesting to read. Try and think like one of your readers, Is the article striking home or is a mindless rant does it have a good to action and make you want to know more by clicking your link, and would you be interested to read it if you where reading up in that niche.

I pay around $5 per article but I order in small bulk eg 50 sets at a time as well as making sure that once you find a good writer you stick with them or at least get a good hand picked team of writes that you can work with :-)