Dropbox is free, and they're actually providing something that costs money (bandwidth and disk space in some data center holding copies of my data). Your service would use my own computers, my own bandwidth and my own disks. I wouldn't personally buy it, as I already have my own little scripts that make mirrored backups each night. If I were to find some feature of yours so compelling though, it'd be an "app store" type purchase -- $6.99 or less one-time fee.