The drive themselves might be an extra 10 or 20% of the cost of the machine. They come with other factors as well. 5x the disks means more power draw and coolin

The drive themselves might be an extra 10 or 20% of the cost of the machine. They come with other factors as well. 5x the disks means more power draw and cooling. It probably also means you need a 2U chassis instead of pizza-box 1U, which means your datacenter rent doubles. (It may be possible to fit 10TB into a 1U. I've never tried it, but I suspect you'll have space / power / heat problems.)