I would say that a amount sounds more than reasonable.
I guess the real point here is that you should work out your costings in advance (I use an excel spreadsheet for different kinds of work) and let them know upfront how much the work will cost.
Add in an hourly rate for amendments that the client wants to make.
I've had the situation myself where I've cracked on and ploughed in loads of hours, only for the client to either change the brief or question the invoice.
Communication and transparency from the start is the best way to avoid situations like this!
