NAS (Synology DS723+) - £450 1x 8TB Iron Wolf - £161 (£322 for RAID 1)
Wasabi Sys charges me $6.99 per TB (I pay around $8 for my critical data only) Hetzner Storage Box is ~£11 per 5 TB (I backup my critical + non critical data here)
So after the initial cost of the NAS and the drives, I spend £18 a month for backup storage.
I am not including the electricity cost because the NAS is pretty efficient. (21 watts during data access)