Your $1500 includes all the overheads that a storage provider amortizes over their entire customer base. At scale, the cost comes down to the cost of the hard drives, plugging them in, keeping them on, and bandwidth. The margins are really thin, so you need to reduce costs further by doing things like powering down drives that are not in use, or leaving failed drives in racks until they can be replaced in bulk. And importantly, if you offer something unlimited, make sure that it is actually limited. In Backblaze's case, the only 'unlimited' offering is only for their custom backup software that only backs up a single PC, and more recently allows limited backup of external drives. You can only keep multiple TB in it if you have multiple TB powered up in your PC and keep your external drives attached enough so their backups don't get wiped. It is impractical for archiving and abuse. And if it was a problem, they just need to tweak the terms of service to disallow abuse as is traditional.
Their price may well be too cheap. Maybe they make a loss due to unexpected costs (thin margins == risk). Or maybe they are priced too cheap, because they know increasing prices will lose even more money by losing more profitable business than unprofitable.
