As crazy as this sounds, the best way to find this is to check out what your competitors are charging. Somewhere along the way, you’ll notice the trends and figure out what the going rates are. From that, you can set and adjust your own rates accordingly.
The one thing that I’d advise is to be comfortable in what you charge. You should be able to justify why people should be paying you what you want to charge them (and no, this isn’t just listing the certifications you possess). You have to be able to sell yourself to your clients, and when you know what you are about and what type of clients you want to work with, then it becomes easy to justify your costs. Hope that helps.