Whatever you decide to charge...be sure to set up a contract form which spells out exactly what is expected by both parties...

things to include...

Scope of the project - exactly what they want and what you will provide
time frame - How long for each phase
responsibilities - who's providing hosting, content etc.
Extra charges, Contingency funds for intial design
Completion point - just what is complete
Post completion fees - what you will charge for updates, modifications
Liability - ie protect yourself if the customer decides to host child porn or terrorist crap


And the big one....
Who owns what - is the code you create theirs or yours

i can't stress how important this is...no big deal if you are designing as a hobby...but if you plan to make a living at it...it is an absolute must...