Some of the prices posted up here are way too cheap - £1500 for 100 hours work! That's fine if you are doing it in your spare time and have another means of income but it equates to an annual salary of about £10,000 for a freelancer.
In my view you need to charge bare minimum £30 an hour if you're working self-employed, but don't just take my word for it.
There's a really useful article here:
http://www.allfreelancework.com/articlenigel5.php
Which links to this handy hourly rate calculator:
http://www.tortorelladesign.com/calc