Buy Storage Pods, designed by BackBlaze. You can get 270TB of raw storage in 4U of rackspace for $0.051 per gigabyte. Total cost for half a petabyte of raw storage: $27,686. To back it all up cheaply but relatively effectively, buy a second set to use as a mirror. $55,372. For use with off-the-shelf software (FreeNAS running ZFS or Linux running mdm RAID) to present a unified filesystem that won't self-destruct when a single drive fails, you'll need to over-provision enough to store parity data. Go big or go home. Just buy another pod for each of the primary and the backup sets. Total of 6 pods with 1620TB of raw storage: $83,058. Some assembly required. And 24U of rackspace required, with power and cooling and 10Gbe ethernet and UPSs (another 4-8U of rackspace).
Expect a ballpark price of something a little under $100,000 that will meet your storage requirements with sufficient availability and redundancy to keep people happy. It will require 2 racks of space, and regular care and feeding. Do the care and feeding in house. A support contract where you pay some asshole tens of thousands of dollars a year to show up and swap drives for you is a waste of money. Bearing that in mind, as other posters have said, talk to storage vendors selling turnkey solutions. Come armed with these numbers. When they bid $1 million, laugh in their faces. But there's an outside chance you'll find a vendor with a price that is something less than hyperinflated. Stranger things have happened.
If you don't generate data very quickly, you can ease into it. For around $35,000, you can start with just 2 pods and the surrounding infrastructure, and add pods in pairs as necessary to accommodate data growth. Add $27,000 in 2 chassis next year to double your space. Add $26,000 of space again in 2017 and increase your raw capacity another 50%. (Total storage cost using BackBlaze-inspired pods is dominated by hard drive prices, which trend downwards.) When you find out your users underestimated growth, another $25,000 of space in 2018 takes you to somewhere in the neighborhood of 2 petabytes of raw storage, that you're using with double parity and 100% mirrored backup for a total effective useable space of approximately 918TB. You'll be replacing 2-3 drives per year, starting out, and 0-1 after infant mortality has run its course. Keep extras in a drawer and do it yourself in half an hour each on a Friday night. If you configured ZFS with reasonably sized vdevs, (3-5 devices) the array rebuild should be done by Monday morning. By 2020, you'll be back up to replacing 2-3 drives per year again as you climb the far side of the bathtub curve. While you're at it, you can seriously consider replacing whole vdevs with larger capacity drives, so your total useable space can start to creep up over time, without buying new chassis. By 2025, you will have 8 chassis in two racks hosting 2.88PB of raw storage space that's young and vital and low maintenance, having spent roughly $200,000.
A bargain, really.