After all, this is someone who has gone to the trouble of getting in touch with a professional and asking questions. They aren't asking their nephew to do it.
They know it's going to be more expensive, but they didn't realize HOW MUCH more expensive, and that comment is simply part of how they learn why that's the case.
They generally honestly don't understand why it's so expensive -- because they don't yet understand what they're buying (it's non-obvious!) -- so the best route is to keep the tone pleasant ("ha! yup, I get that question a lot") and educate them in a friendly way.
The end result may be that they decide they won't want a professional website -- that's fine. Part of the discussion should be how likely it is that the site will earn them back more than they're paying for it. I have a neighbor that wanted to hire me to build a website for her small-scale manufacturing business, and through our discussions we basically figured out that it wasn't going to be a good investment for her right now. That was a win-win situation, in my book.