Hi Sasha,
To be honest, deciding what to charge is probably the hardest part of starting out as a freelance writer. Depending on the type of writing you do, the range can run from $25/hour for content and catalog work to upwards of $10,000 plus royalties for a sales letter. After years on my own as a copywriter working for both large corporations and my own clients, I've definitely learned a few things...
The biggest is that charging by the hour is not the best way to go unless you're working contract gigs for larger companies. It's much better to charge by the project for a few reasons...First, you really don't want a business where all you do is trade hours for dollars. It's about your hard-earned expertise too and the value you bring to the table. Second, it's better for your clients if they know what your services will cost up front. Third, there are a lot of other aspects of your job and taking care of your clients that you need to be compensated for other than just the time spent at the keyboard.
Finally, it all goes back to the old story of a woman who sees Picasso in a cafe, and asks him to draw her something...anything. So he grabs a napkin, sketches on the back and hands it to her. Then says "That'll be $3000." Shocked, the woman says, "But it only took you 30 seconds." And her responds with, "No it took me 28 years of study plus 30 seconds."
When you charge per project, I've found it best to base my fees on my day rate. Coming up with your day rate is the tricky part. If you're writing content and press releases, you usually can't charge as much as you can if you write direct response and marketing copy (my line of work). Because the value of good marketing copy (IE getting new clients in the door) is more obvious.
When I have done press releases, I make it clear that there are no guarantees as to whether or not it will be published. And that has no impact at all on my rates because I always do the best job I can and I have no control over what editors choose to run with.
If you do a quick Google for copywriter's rates, you should find a couple of different, helpful guides available. Then I advise charging just above the mid-range. That way you're not the most expensive, but you're seen as better than average.
Hope this helps...
Here's to your future success!
Stacy