Posting late, but I've found a system that I'm happy with, so thought to share.
I use a combo of on-site external drives and off site backup.
I have music on 3 drives here at home, all with spinning disks. 1 backup is the same as no backup, therefor 3 drives. Every month I thest each drive to make sure there's no bit rot, and every 5 years or so I get a new set of drives. Currently, I use 3TB Toshibas, which cost around $80 each.
I didn't see much discussion about issues with location of backup in the thread. What happens if there's a fire/flood/smoke? I subscribe to Back Blaze for around $60/year, because I have photos and other things I treasure in digital form. One of my 3 drives stays mounted all the time, and is backed up to Back Blaze automagically.
Current cost is roughly $110/year, but I already was a Back Blaze user so not sure whether I should have included that in the total.
~Chris