The most reasonably priced SD cards you're probably going to find at the moment are 64GB; 2x64GB (£17 ea,
, £34 total) is still cheaper than 1x128GB (£47 ea.
). To achieve 8TB using 64GB SD cards you'll need 125 of them; using the ones linked above that's a total cost of £2,125.
Assuming you could build/assemble some kind of USB controller which can simultaneously read/write to 125 SD cards at full speed (which is the sort of engineering challenge that would land you any electronics engineering job in the world you could possibly want at whatever wage you choose) you'll still have problems with flash wear: flash memory cells will wear out far faster than a traditional hard drive. And, of course, it's not terribly cost-effective.
Alternatively you can get 3.5" hard drives rated for NAS use for £115
each. For the same price as two of those drives you'd only be able to get 832GB in SD card storage (13x 64GB SD cards). External SATA USB enclosures are relatively cheap, ~£10 to £20 at most, so you wouldn't have to build your own controller array.
More to the point though, I don't think the Pi is best suited for high-throughput NAS duties. The maximum throughput for USB2 is 480 megabits per second; the latest SATA version tops out at a maximum throughput of 6 gigabits per second. You're not going to get the benefit of the hard disk speed because you'll be limited by the USB bus on the Pi. You'll probably never achieve the maximum theoretical throughput because there will be all sorts of overheads which will impact the data transfer rate. On my home network I get a maximum transfer rate of about 100 megabytes per second (800 megabits per second) when transferring large files to my network server over gigabit ethernet; that's measured "real world" performance, not theoretical. I'm only using SATA 3Gb/s in my network server, and the real world performance is nearly twice as fast as what a USB2 port can theoretically
You'd be better off getting a dedicated NAS unit or building a network server out of cheap second-hand parts. For reference my network server is based on an Intel Core2Duo E5400 processor - relatively old hardware, and really cheap second-hand, but it still performs more than admirably.