It's one of the most common questions posed to successful freelance writers like myself. The truth is, if you start out the way I did, answering ads for bulk content writers, you won't get to set your own price for at least a few months.
Without a decent portfolio and the confidence that comes with it, freelance writers are usually forced to agree with the terms set out by the client. The time it takes each writer to put 500 perfect words on the page varies, but I think it's safe to assume that no less than $30 per hour of work is acceptable.
Don't worry, because in no time at all, it will be $50-$75 per hour!
Like I've said before, every freelancer needs to build up a decent portfolio before he or she is able to set a price per hour, or per project. How do you do this? By accepting lots of work that doesn't pay what you really want - or even need, sometimes. Within six months, if you are serious about your freelance writing career and not completely inept at producing the written word, you'll be able to move up in the industry.
How much to charge for freelance writing work also depends on the type of work you're doing, as well as the clients you're negotiating with. Startups will inevitably pay less than established companies with a million dollar annual profit line - that's just the way it is.
Once You Are An Established Freelance Writer - What Then?
When you've paid your dues and put together a decent portfolio, it's your time to make the rules! Choose a specialty, write your budget, and think seriously about what you need to earn per hour to make this worth your time. This isn't a hobby, after all!
Good writers should be making at least $50 per hour, in my furry opinion. If you can make more, by all means ask for it! The funny thing that I've realized throughout my own career is that when you ask for too little per hour, clients are more likely to badger you, criticize your work and ask for multiple revisions. When you ask for a decent hourly wage? They tend to trust that you know what you're doing, and leave you to it.
I don't know, humans don't make a lot of sense sometimes. We've just got to work within their system and turn out some good product!
Further reading: 6 Tips for Building your your Freelance Business