Now I firmly believe that when you quote a price, you stick with it. But what happens when a client asks for more?
The answer is simple... re-negotiate your fee. It's fair and reasonable to do this and as long as you are up-front there will be no surprises or ill will.
To be a successful freelancer your job is made easier by return customers. Say goodbye to recurring customers if you take advantage of your client. But you don't want to be taken advantage of too.. so speak up.
I have found that my clients understand that if they are asking for more or if they have changed the scope of the project, that an adjustment of the costs is appropriate.
Bottom line: Be fair to yourself and your client. Get more if they ask for more.. but don't surprise your client.. work it out ahead of the time.