I recently came across a really great website, with a wealth of information for freelancers. Amongst other topics, there is an excellent salary calculator tool, which enables you to input lots of information, such as what your expenses are per year (inc cost of any computer equipment, travel for work, phone costs, electricity if you work from home, etc), and balance this with the number of hours or days you generally do, and it works out how much you should be charging for your hourly rate. Try it out - it's really useful. The website is: www.freelanceswitch.com
It calculates the amount based solely on the factors you put in, and is generalist, so it clearly can't work out exactly what might be right for your industry and area of expertise, but it will give you an indication of what you should be looking at to cover your costs and make a profit. Let me know if it's any help.