With websites such as tutsplus.com paying their guest bloggers $50+ per article, and there's a whole lot of other places where people get paid to guest post for more than 50 bucks.
So if I was to buy an article I would work with someone who charges $25 or more for a 1000 words article. I say "I would" simply because I prefer to write my own content, every time I buy an article I end up editing it which was something I was trying to avoid when I was buying the article.
But instead of going to freelance I would go with someone who gets paid as guest blogger to write my content. Go to google and search "get paid to guest blog" or anything similar to that then visit any website and see who has the most popular posts and negotiate with them.
Most guest bloggers will even promote your post/article to their followers as well, so it's a win-win.
Hope that helps.