It really all depends on what they want done and how much work it will take. Then after you consider all that, you determine how much of your time you will spend on it, and any costs that you might need for the site (if you purchase their domain name for them, if you purchase their hosting plan, etc)
Basically, its a factor of how much you will NEED for it + plus working charges.
Working charges are determined by how much YOU think your time is worth, if it is going to be a difficult site to put together you can charge more (if it requires CGI, PHP, FLASH, mySQL, and other various scripting on your part), but if its simple, then you can charge less.
I guess what I am trying to say is, it’s up to you, but don’t sell yourself short, first determine what they want, then figure out cost.
I hope this was at least of SOME help (which is probably wasn’t, and if it wasn’t, sorry to waste your time)