I still think tape is cheaper for any type of off-site, archival mechanism. Perhaps HDDs can fit, but most places I know can't get all their backups off to disk, in a reasonable time, at a reasonable density, for a length of time. In a few larger organizations, we've run 2 copies of tape to keep one around locally, one off site for the fire/other disaster.

Maybe the last few years, with the cost/GB of disk crashing, that changes, but I'm not sure. I would really love to see more research here.

Tapes are offline, physically moved. Disk isn't necessarily.