can I use my own domains on iCloud for $10/ mo. ? Are there extra charges for each domain?

Apple charges nothing for using custom domains. But you must own those domains and thus must have them registered with a domain hosting service, who will charge you for that account and those domain registrations.

Apple is not a domain hosting company, so they can not provide you with hosted domains. You need a hosting service account for that - e.g. GoDaddy, IONOS 1&1 or some such hosing services company account.