Hi @Lieven Lannoo and welcome to Community!
Since you mentioned Aura and Karma, you should know that for a site builder experience, you should combine them with Spacecraft. You'll have a nicely themed website experience, with all the content managed in Confluence. Spacecraft allows you to create public sites, like for documentation, but you can also keep the sites private and require Confluence login for all your internal knowledge management.
Mantra offers a social intranet, with a news feed, blog posts, internal events, polls, shout outs, that sort of thing, but it's based on participation (employees can share news, start polls, organize an event, etc) so if you want a read-only option I don't think Mantra would be a good match.
You mention pricing for one user, but on Cloud you will need to pay for all your employees who are using it, or have at least 1 paid user for each 5 guests. The only option to only pay for one user would be for the Confluence Cloud space(s) then to be open to anonymous users (the whole internet, beware).
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