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.)