How much does it cost for your own domain?

I do not know what you are asking. this link - Use Custom Email Domain with iCloud Mail - Apple Support says you need iCloud+ to use a custom domain name.

When you subscribe to iCloud+, you can use up to five custom domains to send and receive email with iCloud Mail.  Learn how to get started.

You need iCloud+ to create a custom domain so it does cost whatever that amount is.

What does that mean, please explain.