Given someone already pays for electricity and internet at the location, I'd say the cheapest option would be to ask all the members if someone has an old laptop to donate, maybe even with a broken display or whatever, main thing is it still somehow runs. Rip out the battery, Install Linux, Nextcloud (maybe Yunohost), and put it somewhere without public access. That'd be entirely for free, minus the work to set it up and maintain it.
My smaller VPS costs somewhere around 70€ a year, guess that could be worth it as well as long as it contributes something meaningful.
And be prepared to be disappointed, 99% of my scout group never used the selfhosted services I tried. I guess that's somehow okay. They were focused on the real life activities and no one had any interest to do office work or remember logins... Was always the same 2 people who did paperwork and they didn't need a cloud, so I scrapped it. Your story could be different, I'm not saying it needs to turn out that way.