You cost depends if you want anything on your website or not. Buying a domain is typically USD $12/year. On top of that then you have 2 options for emails:
You can use Gsuite or Microsoft 365 for email hosting. I think both options there are around USD$5 a month. This cost is typically for 1 email address only.
This route I would purchase through Google Domains.
If you want something on your website, then you need web hosting which will provide email hosting free. You can have unlimited email addresses with this option. Website hosting runs around USD$12/month
I recommend Siteground. This option I believe will also provide your first year of domain registration.
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