What are the hidden costs of websites?

That's a really good question, because with any process as complicated as making custom websites, there are inevitably changes that nobody can predict that will affect the price. Some areas to be aware of are:

unplanned rounds of feedback and revision

additional features or sections added to the site

hosting and related technical costs

website software costs, like the theme or plugins

outsourced marketing services like SEO, social media, etc.

outsourced content providers like copywriters and photographers

Ultimately, the website designer / developer / agency should discuss each one with you, and together, you should decide if each of these "costs" should be included in the original quote, or if they should just be added at the end if they happen to be needed.