The biggest factor is the number of pages and how many of them are one-of-a-kind. Ten sub-pages with a similar structure are cheaper than three that each need their own design. The second factor is content: if the texts and images exist, the work goes faster; if they have to be created, that is a separate job.
The third factor is functionality. A contact form and a news block come with the package; a booking system, a customer account, multiple languages or an integration with accounting or inventory software are each a project of their own. The fourth is multilingualism — every additional language means both translation and separate SEO work.
The cheapest website is not the one with the smallest invoice, it is the one that brings in customers. A €500 site nobody finds costs more than a €1,500 one that brings two enquiries a month.