I mean, if you want a full mirror, rolling your own backup target is going to be the cheapest option even with the current high price of hardware. Other options are cloud storage, or using another media like tape. Cloud storage is of course an on going cost which rules it out for me, not to mention privacy concerns. There are certain “cold storage” options from cloud storage hosts which are considerably cheaper but they have limitations on how the data can be accessed and how often. The tape route is possible but it’s not really viable for home use due to the high upfront cost of the drives. Outside of that, backing up a subset of your storage as others have suggested is the only other option. Creating viable backups without breaking the bank is a challenge as old as computers, unfortunately.