I also think they made a mistake, but it doesn't look like a huge one?
4 MB ~= 4000 kB, (4000 kB - 450 kB) * $100,000/kB/year = $355,000,000/year
(With a bit of fiddling to get the same answer as them, I think they may have done this: (4000 kB + 350 kB) * $100,000/kB, though I wouldn't want to guess why this error happened).