I worked out the math a while ago and it will take over 10 years for a home-built NAS to be more cost-effective than iCloud, and that's only if there are no hardware or disk failures, and doesn't include the additional hassle of being out of Apple's model when it comes to auto-backups and storage.
It depends on what you value. If you value keeping your data at home, and accept the trade off of higher cost and the need to maintain it yourself and use some sort of sync mechanism that isn't built into the OS, then you do that.
