1.5Tb external drive = $138 (Western Digital Elements @ Dick Smith)
2.0Tb external drive = $187 (Western Digital Elements @ Dick Smith)
100xDVD about = $40 (@ Dick Smith)

Double everything (for redundant backup).

Hard disk method = $(138x2)/1500Gb ~= 18.4 cents per Gb
Hard disk method = $(187x2)/2000Gb ~= 18.7 cents per Gb
DVDr method = $(40x2)/(100x4.7)Gb ~= 17.0 cents per Gb

Note I upped the DVDr method to 4.7Gb per DVD (I was previously calculating on 4.3Gb). But, to be fair though, you rarely fill a DVD when burning a backup.

Also...

The "1.5Tb" drive isn't actually 1500Gb. Western Digital use the "industry standard" of one terabyte (TB) = one trillion bytes, when it really should be 1,099,511,627,776 bytes (according to Google).

Meanwhile, a 4.7Gb DVDr can store 4,707,319,808 bytes, which is actually 4.384Gb.

So if we calculate per billion zillion trillion gazillion thingies:

(BB = billion bytes)

Hard disk method = $(138x2)/1500 BB ~= 18.4 cents per billion bytes
Hard disk method = $(187x2)/2000 BB ~= 18.7 cents per billion bytes
DVDr method = $(40x2)/(100x4.707)BB ~= 17.0 cents per billion bytes

Oh... that isn't any different. Yay.

Meanwhile, I've finally put my money where my mouth is and bought two 1.5TB drives and am using them both for redundant backup instead of DVDrs. They should last "forever". We'll see...