I have not bought that many articles, but my results have mostly been very good. I've also found that in addition to voice and audience, it helps to stipulate the format. If the writer is a bit savvy about the web, saying things like
"Articles should be broken into 2-4 sections with subheadings. One section will consist of 3-5 bullet points"
Has gotten me articles that come closer to the finished product. I've gotten some excellent, but somewhat pricey authors off Elance, and some very good articles from.... I think it's called SEO Article Pros??? I can look it up if anyone cares.
If you buy in bulk, they'll be down around the 2 cent/word range I think
Check:
http://seoarticlewritingpros.com/order/