For US writers the rates by the word vary widely from .01 to .25 cents per word. Of course well paid authors get paid even more than that.
There isn't any fix rate for freelance writing. Different companies offer different rates on different articles. The rate also depends from country to country. For example, in most Asian countries, writing a 600 word article can earn you only 2 dollars. The rate would be much more in UK and America. If you are a good writer then you can even earn more.