Running an ordinary wiki costs nothing. A wiki hosted on wikigit.org runs on its free hosting, and if you keep your pages in your own GitHub account, that’s free too. A very large or very busy wiki may eventually want a paid arrangement, but most never need one. And because your pages are plain files you own, your content is always yours to export or move.
This note appears on more than one page — it’s a single page included in each, so editing it here updates it everywhere.