Depends what you mean by 'reasonable' but I've always been happy to pay Hetzner c. €22 per month. You do get a lot of storage with some of their servers.
If it's just a sync backend then does that matter?
I recently started using a Digital Ocean droplet for my Nextcloud server. It seems very nice so far and I love the PhoneTrack addon. It helped me map out where we're going to put some of our buildings on our property.
I am going to look into mouting my home server's 2TB ZFS array via NFS on my droplet this week, and then reinstall Nextcloud and use that mount point for the user data.
I use GCE (Google) for this purely because they had the lowest per-byte cost for block storage, at least when I looked. The instance itself has the smallest CPU/memory config. IIRC my bill is about $7/mo for the instance with separate 200GB volume.