If you went with the solution that BackBlaze rolled for themselves, you'd be looking at 15 of their servers (for redundancy), without even adding any more data in the future. Using the solution they came up with, that would run almost $120,000. I have no idea what the monthly maintenance expenses would be.
http://blog.backblaze.com/2009/09/01/petabytes-on-a-budget-h...
You are undertaking a nearly herculean feat, here. I don't think any of the professional data storage solutions out there will be any cheaper, either. In fact, a petabyte of storage (especially if you need doubles of all hardware to place at another location) would probably run nearer a million bucks or more.
It might even be worth, as someone else linked, considering BackBlaze themselves. At least find out what they could do for you. They offer a business plan at something like $50/computer for unlimited storage. Obviously they're not going to store 0.5-1.0 petabytes of data for that price, but on the scale that you need, you might be able to negotiate something that surpasses what you could accomplish at any reasonable price -- and you have the added benefit of any catastrophe being their problem; not yours.
If you truly only rarely need to access your data and speed of that access is trivial to you, then as others mentioned, Tape could be completely feasible. I have no idea how much money it would cost, but you're talking a TON of cartridges and some sort of rotational system. If you expect the amount of data you need to continue growing at a rapid rate, you might even end up considering something like Spectra Logic offers. I have absolutely no clue how much their solutions are, but they can store up to 91 petabytes of data which I imagine would do you quite well for a long time to come:
http://www.spectralogic.com/index.cfm?fuseaction=products.di...