When making the site, who will be supplying the content? Also with Joomla some basic functionality needs extensions that normally need to be purchased or come with links to the devopler which you should take into account.
How many pages are they requiring you to do before the site is approved and goes live? I have found it is best to give a rough guide of what they will have at the end of it, if you say £500 to make the site the customer may expect a lot more than you are planning.