How much you should charge a client is an extremely personal decision. It's best to start honestly answering a few questions.
- What is your skill set? Can you program, are you a dreamweaver pro, or are you a wiz with CMS' like Joomla?
- What is your experience? Here's the reality, Web Development is easy to get into, but the way Web Development industry is structured, there is a very sharp learning curve if you want to make some real money.
- What is your proficienciey at these skills? If you managing a 2,000 product catalog and your client gives you 2000 pixel product images how are you going to resize them?
- Do you have a portfolio? This is along the same lines as if you have any experience. However, if you have just done websites for your friend's quiltting business (nothing wrong with quiliting) then you're going to have a hard time convincing the CEO of a restaurant chain that you've got what it takes. Even if you really do.