The first cost of running a WordPress site yourself is your time, and it never shows up on an invoice. Choosing a host, installing the software, comparing themes, configuring every plugin, then coming back each week to check that everything still works, that's hours you don't spend with your customers. The official wordpress.org documentation advises backing up the site before every update, then reactivating the plugins and checking that they work afterwards. None of it is hard, but it needs regularity, and regularity is exactly what goes missing when business is good and the website slides down the list.
The second cost is maintenance, meaning the updates to the core software, the theme and each plugin, which all move at different speeds. Since version 3.7, the wordpress.org documentation explains, minor and security updates apply themselves in the background on most sites, but major releases still need a click from you, and nothing automates compatibility between two plugins written by two different teams. A contact form that stops sending messages after an update often goes unnoticed for weeks. I've written down what real website maintenance covers in a separate article, and the list is longer than it looks from the outside.
The third cost is security, and I'll stick to the facts here. The WordPress.com comparison page notes that on a WordPress.org site, security depends on your host and hosting package, and that installed plugins could, depending on the plugin, compromise your site. In practice, a site with one plugin left without updates can be used by someone else to publish pages that aren't yours, often invisible from your homepage and yet indexed by Google. The owner usually finds out through a warning in the search results or a phone call from a customer. It happens to sites of every size, and the cleanup takes days nobody had planned for.
The fourth cost is the total over three years, which belongs on a sheet of paper before you compare anything. A WordPress site you run yourself adds up hosting, the domain name, a paid theme, a few paid plugins renewed every year, and above all the hours spent or the invoices from a provider when something breaks. Each line looks small, and it's the sum that surprises. On the other side, three years of Essential come to €1,405 excluding tax, everything included, with no extra invoice when an update goes wrong. The pricing page details both plans, with the first year total shown at the top of every quote.