Can we presume this event did not happen? The title is "hypothetical," which means the situation has been framed as something that could happen, but did not. If it did happen as stated, then it is not "hypothetical," rather reality.
If it was reality, then the last comment was on target, "... it is your fault for not being specific enough." No, you may not know exactly the time and cost the customer will be charged, but you should be able to provide a range, and get an approval for the range. The $30 and $400 is a huge gap. If a range of prices, with a minimum was given, then you and the customer had common expectations. Without having given even a range of prices, the expectations of a final cost was not even established.