My Constant-Content articles sell for $48 a piece...of which I net $30. Those are 500-750 words. I set my own price, and don't lower it, although I do allow clients to make offers (which I refuse, because they are always low-ball. If they really want the content, they will pay full price).
As another member stated, your grammar must be flawless. Not only that, but you must have an intuitive grasp of proper paragraph structure and article flow. You can have perfect grammar and still get rejected because the article is all over the place. This is why Constant-Content has customers who will pay professional rates. Customers can go onto the site and know from the get-go that whatever they encounter on the site will be top-notch.
You must also have a good sense of which topics are popular online. Some topics are "evergreen," meaning that they're always in vogue. Tech, health, fitness, finance, etc. If you write articles on laughing kittens and price them at $40+, they will never sell.
There are people who sell articles for $100. This isn't a myth. I doubt any 500 word articles sell for that rate, but if the information is extremely valuable, current and well-referenced, I'd say it was possible. I could see charging $100 for a 1000 word article if the information within isn't available elsewhere in a concise form.
one more thing: if you want to sell at professional rates you must remove all fluff from your writing. If you don't have plenty of facts to relate--facts that will take hundreds of words to get across--then you don't have an article. If you suspect you have a problem with fluff, then you do.