The hidden costs of building any website are addon services such as email marketing services, business phone services, a professional business email address, SEO tools, and other third-party extensions / apps that you may need (varies based on the type of site).
We have written a detailed guide on how much it costs to build a WordPress site (with tips on how to keep it low budget).
Other typical hidden costs in website builders are storage space, bandwidth, domain renewal costs after the first year, higher renewal pricing on some platforms, and aggressive upsell from the sales team.