Looking at the parts list I see the largest cost is the 2x 40-port SATA cards (aside from the drives). I wonder why they didn't decide to reduce the number of drives to 40 so they could just use one SATA card and save $688?
Let's take a look...
They do have an oldish post on hosting costs: http://blog.backblaze.com/2011/07/20/petabytes-on-a-budget-v...
A rack in 2011 cost them $2,100 per month (it's more now, guaranteed). Their racks look to hold about 11 machines. Each pod is then about $191/mo.
Each rack thus needs 1.375 extra pods at "low density". That's $262.50 a month. Since you now need about 10% more pods, you're paying about $29 more in hosting costs per month per pod.
Your savings are not $688, since you also need to construct 1/8 pod later. A low-density pod will cost about $2719, with is $340 split 8 ways. So real hardware savings is $348. Hosting costs eat that in exactly a year, and they are certainly expecting a longer life than that. (That's 2011 hosting costs, which are certainly lower.)
So it's worthwhile to spend a little extra for the additional drives. (Of course, if they could get the extra five in with more modest hardware, as seems quite possible, that would be a sure win. But sometimes ease of development/assembly/spare parts wins over hardware cost.)
Still, at least 12% extra rack and floor space to get the same storage capacity would probably cost them more in the long run.
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