The problem with the math is because you really need to factor in the capex cost of the NAS and the opex of a larger number of hard drives; the cost-per-TB of the larger drives isn't as much as you might think. What you're really looking for with a NAS device is to be able to store XXTB of data, and worrying about the pricing of individual components isn't always necessary or meaningful.
If my 6TB-drive-based NAS platform costs $750 for the NAS and $250 each for four 6TB drives in RAIDz2, then my total cost is $1750 for 12TB, or $145/TB.
If I build a 3TB-drive based NAS platform out of $100 3TB drives, there are two ways that can go:
I can build it with 4 drives (keeping the number of drives the same). That costs $750 for the NAS plus $400 for all four drives, or $1150 for 6TB, or $191/TB.
I can build it with 6 drives (matching capacity by increasing drives). That costs $750 for the NAS plus $600 for all six drives, or $1350, or $112/TB. That's actually a good deal, but it increases the number of drives and therefore the power and also trades off future expandability (less space in the case, etc).
