Not too terrible compared to hard drives, per TB. I would use them mostly for long term archival though as they're not really meant for continuous writes. Although the cost effectiveness only really starts to make sense after you have maybe like 100TB of data which I don't, so it's one of those things I kind of want but also don't really need.
I have lot of oddball small hard drives, need to come up with a backup solution that can span jobs across multiple media, then I will use those drives the same way you'd use tapes. Once I exhaust that, then I will probably buy a tape drive.