For me, it depends on what site I'm using it on. Landing pages I usually write myself by compiling all the best info from all my competitors existing landing pages, filler content for my bigger sites around $4-5 per 100. For my toss away sites, I usually just plop down an unreadable but unique spun article, and once this spun article starts to rank and I see people coming in, I swap it out with a $1 per 100 word article.
For SEO purposes, I usually pay around $0.6 per 100 words. And sometimes if I find a writer who can write engaging + informative content, but is not a native English speaker, I just spend about 15 minutes and fix up the article myself and put it as filler for one of my sites.
For those who think quality content is the only way to make money, that's only half true. It depends what you're promoting. For adsense and amazon, the quality of content doesn't seem to make much of a difference from my experience (as long as your site design/navigation looks nice). But for clickbank and other affiliate programs, it makes a huge difference.
Depending on your niche, give iwriter a shot (most writers there suck at writing review type articles though). It's a hit or miss, but when you get a good writer there that's just starting out, you can favorite them and get a bunch of high quality articles for cheap. Oftentimes, I even get better $3 articles from them than most of the BST services here that sell at $1 per 100 (at least the ones that probably outsource to odesk for $1 per 500 words based on some of the quality I've seen).