I like the approach taken but the scale is simply unrealistic. For the sake of the argument lets assume the chips (silicon & packaging) is possible at $0.20 per GH/s ($20 per TH). Small runs (less than 1,000 wafers) at 28nm are probably more expensive than you think but lets ignore that and look at the rest of the system.
At the network speeds we are talking about, those wouldnt be small runs. As for the price per chip, these bitcoin asics are no bigger than typical mid/highend mobile SoCs. You've already seen how I came to my estimates, but lets see what ARM has to say on that:
"Our SoC partners might put the first one out on the market with a $15 ASP, and as soon as the second person comes on the market the price drops to $7 and within a month or two, they'll be at $5."
http://www.theinquirer.net/inquirer/news/2303601/arm-reveals-the-mali-t720-gpu-to-help-vendors-get-android-devices-to-market-faster
In all likelihood, mobile socs would have lower yields (far less redundancy), more layers, and far more complicated IO. On the flip side, packaging for low wattage might be easier, but if HF could save money by using seperate chips per die, rather than packaging 4 in one MCM, Im pretty sure they would do it. Really, your low estimate is pretty high. Maybe asic vendors now cant reach that low because they dont have the volume to order directly with the fab, but at some point they will.
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For example lets try to guesstimate the balance of the system (everything but the ASIC) on a Sierra. Here is my guess what is yours?
case: $60 * 1 = $60
case: $60 * 1 = $60
Surely you are joking?
Have you seen the cases of KnC? Just 2 flimsy cheats of aluminum that couldnt cost more than $3 in volume. For reference, here is a full fancy looking ATX case for $6
http://www.alibaba.com/product-gs/1016257579/Mini_ATX_Case_Mini_ATX_Tower.html
IT even comes with fans that you price at $10 each
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watercooling: $80 * 3 = $240
Again, you have to be kidding. Its not because those prices are common in retail, targeting a select niche market that tolerates such prices, that large volume orders would cost anything like that. $20 tops. Here is a huge radiator, likely the most expensive part, for $12:
http://www.alibaba.com/product-gs/425032365/Computer_water_cooling_cooling_system.html
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case fans: $10 * 2 = $20
You are off by, well, a LOT. Again, for reference first hit on Alibaba, pricing starts at $0.6:http://www.alibaba.com/product-gs/1059988680/High_tech_120mm_fan_12v_dc.html
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power supplies: $150 * 2 = $300
PCB (both PCB manufacturing and assembly): $30 x 3 = $90
DC regulators (12V to ~0.8V 200A output ea, 2 per board): $30 x 6 = $180 (probably more KNC uses >$300 per 400 GH/s system)
minor components (connectors, capacitors, etc guesstimate 100+ components per board): $50 x 3 = $150
labor (post PCB assembly, testing, packaging): $50
PCB (both PCB manufacturing and assembly): $30 x 3 = $90
DC regulators (12V to ~0.8V 200A output ea, 2 per board): $30 x 6 = $180 (probably more KNC uses >$300 per 400 GH/s system)
minor components (connectors, capacitors, etc guesstimate 100+ components per board): $50 x 3 = $150
labor (post PCB assembly, testing, packaging): $50
Im not gonna look for prices, but honestly, you are way,WAY off. First crappy 360W PSU I find
http://www.alibaba.com/product-gs/1313266704/360W_Constant_Voltage_12V_Power_Supply.html
Yep, $10 instead of you estimated $150. etc, etc.
I didnt even search for the cheapest, just took one of the first relevant results to give you an idea. Now the above components may be as crap as the infamous BFL power supplies, might be wrong spec, whatever, but you get the idea. You really confuse retail prices of A brands with what stuff from China costs in volume.