first, only bill a set fee if you and the client are very certain of the specification and work required. If not, bill hourly or daily.
I chose the initial amount based on what the salary of my previous job had worked out at daily. Then factor in your reputation - if you are starting out, offer cheaper rates and possibly target poorer clients (charities/schools etc) who can't afford as much, but will be super-enthusiastic about their site, and so spread your name. Once you are established and people start asking for you by name, then you can start charging more.