Hi Jo
Best thing is to be proactive rather than reactive. By this I mean work out your cost of doing business and then add your required profit on top of that. Don't forget to include any rent (only the percentage of which you use for doing business) and your salary etc. Usual profit areas vary and you can base this on your experience, whatever you think your time / creativity / skill set is worth.
Try this to help you out, it's an online 'rate' calculator;
FreelanceSwitch Hourly Rate Calculator
I've not used that particular calculator personally but I'm sure it will give you an idea, even if it's just a start.
Hope that helps,
Steve