If you're just starting out as a writer, base your fees on a comfortable hourly rate -- but add an extra 30 per cent. You add the additional to cover all the unseen costs, such as insurance, including your health insurance, computer update and repair, and so on.
Beware of charging rates for your writing which are too low. Please don't try to undercut other writers -- this never works, it just means that you both go broke. :-)
If you're not sure what to charge, and you're a current or graduate member of any of my writing classes, just send me a message, and we'll work out a rate sheet for you.
I've also covered how much to charge in a new guide, Devilish Writer -- it's opened the eyes of many writers. :-)