Seafile works well as a Dropbox clone. Been using it for a couple years now without much issue. My files are replicated to my main computer and I push files from my phone to the Seafile instance over WebDAV (automatically with Autosync in Android) and download specific files through the app.

Yeah I was more interested in cloud backups than syncing

For that I had to make a custom service that periodically runs rclone. Duplicati does cloud backups, but it has a record of being a bit sus

Is there a server / host that provides storage at a price cheaper than Dropbox though?

Backblaze B2 is the same price as Dropbox for 2TB ($0.005/Gb/mo vs Dropbox $10/2TB/mo)

You can also use rclone to upload to Dropbox as the backup solution. Just wrap it in a crypt target so that Dropbox doesn't actually have the ability to scan your files/lose anything in a security breach