All plugins sold through the Atlassian Marketplace follow the same rule: You have to get the user tier that corresponds to your JIRA license. That's in the Pricing FAQ in the marketplace listings.
For plugins sold by the developer directly, it depends. More often than not however, you still have to get the same user tier as the JIRA install. Otherwise, the plugin developer has to include some sort of user management system where you select which users can use the plugin, and it seems obvious that it's not really in their financial interest to do so.
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