Extra instance is the option which is available only for our premium and enterprise customers. Regarding the cost, please check out our pricing page93.
What is "Extra Instance"?
Extra instance mean to add parallel communities to your main community. All your parallel communities share the same user base. Meaning, people do not need to sign-up separately for each parallel community. Once they are registered in one, they are registered in all other parallel communities too. The admin panel for all these parallel communities will be unified as well.
One common use case is to build multiple versions of the community based on language. For example, you might want your community to be available in both German and English. This way, we will create a parallel community for the German language, something like https://YourCommunityDomain.com68/de.
It means you have two communities, one in English (which is the primary one) and the other in German. But, both are under the same name and domain, however, each portal can have it's own logo and theme.
Multilingual community is one example of using extra instances; it can also be used for other use cases depending on your needs.