One thing you didn't mention in your post - how critical is the data? If you lost it what would happen: Nothing? Would you lose a few hours time recreating it? Would you go out of business? Would you get sued for breach of contract? Would Knees and Knuckles be paying your family a final visit? Knowing that would make a difference in how I would store the data.
As several posters have stated and you've noticed yourself - nothing beats a hard drive for cost/byte.
But then you need to determine what to do with that - do you keep it online at all times (power, space, cooling may become issues to consider).
How many copies of the data do you keep? Hard drives fail. Just because it's raid doesn't mean it's safe.
What's your bigger plan for dealing testing for failure of the backup media and determining when to retire them? Periodic testing has to be one of the most important parts of your plan. You build something test it once and later find out that your last 2 months of backups were worthless. Testing can help you avoid that.
Do you need to keep copies off-site? Having 2 copies in your data-center located in your basement is no good if it floods.
How much total storage do you need, need 6 months from now, 2 years, etc.? There's an interesting article from the online backup folks at backblaze.com. They put together 4U enclosures that store 67 Terabytes for about $8,000 USD. Complete instructions are on their site for how to do it (they don't sell them but use them for their business). However, it's not exactly portable. While not physically huge, it's gotta weigh a bit. Perhaps 2 at separate locations with a network connection between them to keep them mirrored?
There's a number of firms that offer various online backup solutions where your data is automatically uploaded to their datacenter automatically, however I suspect that you're going to exceed their usual offerings unless they have some "poweruser" or business option. For individuals, "$5/mo unlimited storage" seems to be the norm. However, their are 2 limiting factors to that "unlimited" - your/their available bandwidth. If it takes 2 months to push out your dataset - is that acceptable? Also, many firms delete your data 30 days after you delete it. So if you move those hds to your safety deposit box, does the backup co see them as deleted and then delete their copy? Comparing the cost of doing it yourself vs them may be attractive, esp if they have some appropriate business plan that's not much more then their individual plan.
Does the data need to be encrypted? If you loose those hds on the way to your offsite location will it be merely inconvenient or life altering when someone finds it and reads it?
Finally do you need to somehow need to index the data so you can find your backups?
Making backups is easy. Doing it right so you can actually get your data back takes a little more work.
Paul