This is a question which I have been faced with many times - and one I haven't practically found a good solution to. There are many variables that go into the problem; designer experience, future value in website portfolio, value of time, complexity, interest in project, opportunity cost (what else could you spend your time on?) and the list goes on...

Probably the biggest factor in this list is "Time" - afterall other factors such as complexity and opportunity cost can actually be subsumed by this factor anyway. How long will it take you to do the project, and how much is your time worth? You could produce a price from that simple equation. Perhaps with more experience you can add in some value for that experience - i.e. if you can build the website quickly because you've done it before, then up your hourly price. On the other hand, for someone just starting out building websites, don't underestimate the value of adding a project to a portfolio - this can often generate more business in the future... Perhaps this is a way to rationalise an ethical dilemma such as the charity one?

Coming up with a quote for a project is a topic often investigated in a course in Systems Analysis and Design, as there are several academic approaches to it. At the end of the day, the only approach that will succeed is "pricing to win". The only way to win a job, is to undercut the opposition!

I guess you were hoping for an exact figure? Sorry!