This kind of calculation is a nonsense now. A 3TB hard drive can be had for $135 now, vs. a 1Tb three years ago.

These days I've decided I want USB powered drives. Those powered via the wall are too big/annoying.

That brings us back to the same point we were three years ago. A USB powered 1TB drive is around $140. Cheaper on special.

Regardless, DVD is out as a backup method.