I came to the same conclusion. I was aware of rsync.net and tarsnap, and have checked their prices in the past, but for raw storage it's simply not competitive. Some of the other features they offer might make it worth it though if you need those.
Personally i just need a place to dump a backup of my family photo albums and documents. A full backup is around 1TB (deuplicated, somewhat larger raw), and for that there are much cheaper solutions.
Self-hosting at home (or in the office) is a great option for some if you’re not worried about needing an offsite backup. For those that do care about this sort of thing, though, the extra you pay to have someone else manage the thing is well worth it.
Calculation: pi 40eur, 1TB external disk 50eur, typical lifespan of a disk 5 years (when excluding including infant mortality which falls under the mandatory 2-year warranty for new electronics), ~8W power draw is ~€15/year. Let's say you also need to replace the pi after 5 years just for good measure. That's 15+(90/5)=€33/year for 1TB, which gets cheaper per terabyte with bigger or multiple drives.
DIY is often cheaper if you ignore the cost of “doing it yourself” - and securely storing an offsite server is more than just the cost of a disk.
Native ZFS is also a feature for those who can use it.