I just checked prices here in Europe. 3.5" hard drives cost ~€0.20 per gigabyte (with €20-50 for the external casing, bringing an external 500GB drive to 25 eurocents per gig) while DVDs cost €0.10 in 100 DVD spindles - their price has gone down drastically here in the past 6 months. So you were right, DVDs are cheaper. But if one puts any money on time spent backing up and verifying it's a no-brainer.
Since you're using harddrives, you'll be replacing them every three years or so anyway.
And for integrity's sake, let's assume that you're using RAID 1 or 10 (dual DVDs the competition).
Hard disk densities roughly quadruple every three years for the same price at a certain price point (around €100-200), let's assume that DVDs have a similar price curve, but last at least 20 years if stored properly.
If you shoot 10,000 images of 15 MB each year, and that you now currently have three year's worth of images, that's 450 GB of storage, requiring 2x500 GB drives at roughly €140 each. Then you need a RAID controller, that costs €37 for a four-port controller. (Prices from komplett.nl.)
In three years, you'll need another 2x1000 GB drives at €100 each (because prices don't drop that fast). In three more years, you'll need 2x1500 GB drives at €70 each, if prices for drives keep dropping. In three more years, 2x2000 GB at €50 each.
Hardware cost twelve years, presuming that you change no other components: €720.
DVD writer: €28
200 DVDs €174 + 200 DVDs €120 + 200 DVDs €90 + 200 DVDs €70.
DVD cost, presuming that you change no other components: €442.
But the drives use about 1.5 W in standby mode each. Over twelve years, that adds up to about 120 kWh, if they're turned on 25% of the day. Add 25% power supply overhead, and it's another €20.
So, basically, hard drives are not that much more expensive that it would matter much financially as long as we're sticking to a simple mirrored setup. But if you want your working storage to be online and available, the cost doubles again.
The problem for hard drives is remote location storage of backups, where DVDs win for convenience of transport. If you have access to networked storage, hard drives win on convenience, too.
And arguably, it's easier to verify the data integrity with hard drives.
But given the very small additional cost -- both in time and money -- of creating DVD backups as you go, it is prudent to do both: redundant hard drives and redundant DVDs.