You can host Nextcloud at your home using something like NextcloudPi+DDNS (https://nextcloudpi.com) or HomeDrive (https://homedrive.io, which has ngrok-ish end-to-end tunnels integrated), and the monthly cost would be almost $0. :)
Furthermore self hosting a backup server at home doesn't protect against burglars and fires. You'll probably lose both your data and their backup.
That's why I prefer a local backup plus a remote one (a combination of git repositories and file storage. )
3 backups 2 local 1 remote
I've been thinking about this kind of setup for my and my family's data, at least.