When using a hosted website builder platform, the cost of your domain name will vary. It’s important to remember that website builder platforms offer a combination of hosting + domain.
For example, Wix domains cost $14.95 / year, but they are free if you choose the annual plan, which costs $13 / month.
WordPress.com domains cost $18 / year, but they are included for free with the annual Personal plan, which costs $8 / month.
But if you use a self-hosted WordPress platform, then you can start a website with Bluehost for $2.75 per month, and it comes with a free domain name. For more details, see the difference between WordPress.com vs. self-hosted WordPress.org.