There is an upfront cost for the NAS and the hard disks which will vary depending on where you live.

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)

I forgot to include the external hard drive that I back up to once a year.

It was around £150 for an 8TB drive.

Assume a 5-year lifespan (probably longer) with no issues.. (450+322)/(12*5) that's a grand total of 30 pounds a month for storage. Considering the average consumer doesn't use more than 1TB, it's still cheaper at 10 / month and requires zero technical knowledge or skill.