I wouldn't charge less because you did it quicker, when on the other hand you would probably not charge more if something took you longer because you had to fix

I wouldn't charge less because you did it quicker, when on the other hand you would probably not charge more if something took you longer because you had to fix a bug or something with a library you are using. I think if they get what they need all is good. If a client saves 4 times what you charged, then that isn't too crazy. You should capture ALL of the value right? And some companies like Google capture that kind of ratio from all of their employees!