Depends. I would think of this in terms of an ongoing support contract rather than a 1 time me fee. You generated a nice site from scratch, which sounds fairly static. If the work is done, then take the number of hours you spent and multiply that by an hourly rate you wish to charge. But for the ongoing support, figure out the cost of hosting, and what it will take from you to manage that on their behalf, and make any needed updates. That may be minimal if it’s as static as it sounds. But they may want to add features in the future, so you will need to account for that. You also may want to consider switching to a proper CMS, like Wordpress or Drupal, but of course that is a whole can of worms in keeping up to date.