It is nice to have funding, and the amount you need depends on your community organization and size. I host a number of open tools & platforms, all FOSS, and it costs me a few dollars. Something that should be easy to collect via some small donations e.g. on OpenCollective.com. Besides that other FOSS tools are offered as SaaS, either for free or with a free plan, and still respect privacy (and do not bombard you with ads).
You should not make the transition all at once, but gradually. I’d advise to make it a policy to first look if there are any FOSS alternatives that might be useful, and only as second-best choice fallback to something proprietary.
I speak from experience that many people in the Free Software Community (where imho you probably find those most likely to contribute to initiatives such as SustainOSS) all this proprietary use of software is seen as very problematic, and frowned upon.
PS. Maintenance-wise many of these tools are a breeze to set up as well. But you can select on that, based on the technical experience level of the once that will maintain them.
Take for instance Just The Docs → Clone repository, turn on Github Pages… and you’re almost done (do some styling, link to custom domain, commit some markdown content). Slides? → use RevealJS or similar… same story. And they are web-ready everywhere.