First, page count — though not the way people assume. The expensive page is not the tenth, it is the first, because that is where the whole design system gets built. Second, the copy. Supplying it yourself lowers a quote noticeably; having it written costs money and usually produces a site that is found more easily, because the text is written towards phrases people actually search.
Third, languages. A second language version does not double the price; it adds roughly a third, provided it is planned from the start. Added afterwards it costs considerably more, because the structure has to be rebuilt around it.
Fourth, integrations: stock system, CRM, booking platform, payment provider. This is the factor that spreads quotes furthest apart, because it rarely appears in the original brief and only surfaces in conversation. Fifth, the SEO groundwork — which some suppliers price as an extra when it is simply part of the job. A site that cannot be found is not finished.