I have purchased articles in the past. When one pays $2-3 for 450-500 words, you almost always have to do some rewrite to make it coherent. Think of them as a detailed outline and you won't be too far off the mark.
If you want a little quality from a native English speaking person who can write and include some 'meat' in the material you should be thinking 2-3 cents per word.
If you are selling and write quality don't sell yourself short.
the standard rate is $5.00 per 500 words, it it all depends on how you sell yourself. If you can write really well, you could charge up to $10 $20 $25 per 500 words. If you're really serious set up a website with samples of your work.
I started with $1/500 word article. I had no idea that it was a pretty cheap pay for such a lengthy article. But, as I have progressed, I discovered that doing product reviews can earn you as much as $10 per review (600-700) words.
The pay is proportional to your credibility and quality of the work that you provide.