1Tb external drive = $139
50xDVD about = $30

Double everything (for redundant backup)

Hard disk method = (139x2)/1000 = 27.8 cents per Gb
DVDr method = (30x2)/(50x4.3) = 27.9 cents per Gb

But to be completely fair the reality is more like:
Hard disk method = (150x2)/1000 = 30 cents per Gb
DVDr method = (25x2)/(50x4.3) = 23.2 cents per Gb

Still, they're close enough to hardly matter.

The hard part with the hard-disk method is the immediate upfront $300 or so.