That's interesting. I used to sell on-premise Mediawiki support to companies that needed a wiki. Mediawiki is the open source wiki written for and used by Wikipedia.
Obviously there's a bit of a difference in philosophy between Mediawiki and Confluence. But they are otherwise quite comparable in capabilities when it comes to outright information-sharing. Last I worked in this space, there were conversion tools between the two.
Built in mediawiki search is ... well it's already ok, especially if you practice a bit. And you are not constrained to plain search, there are a number of different systems you can use to organize and browse through your data.
Of course there are also a number of semi-external search engines you can add on if you want that can improve results for plain search.
Finally, mediawiki tends to be a fairly rich/high quality data source for general purpose search engines to ingest. If you have a reasonably sized intranet that also happens to have a mediawiki wiki on it, you might end up getting better results for everything, not just the wiki.
That's a really good question. A really large organization might be Eur 2000/month, but 90% of the time that was helping with moderation, which I'd recommend if you start going over Dunbar's number. [1]
For a small org of 50-100 on prem? That's practically idling. Once it's set up it can run almost indefinitely without anyone looking at it all too much. Depends on if you already have someone on site who can watch over an extra LAMP [2] server/vm/[docker] container, run occasional backups etc.
[1] Wikis have interesting properties wrt. Dunbar's number which do not quite fit in this margin.
Obviously there's a bit of a difference in philosophy between Mediawiki and Confluence. But they are otherwise quite comparable in capabilities when it comes to outright information-sharing. Last I worked in this space, there were conversion tools between the two.
[1] https://www.mediawiki.org/wiki/MediaWiki