This is simply nit picking but that little netgear cannot possily handle more then 100ish boards from a single usb port. Not due to
power but the spec of the usb design. So a max of 10 gh/s lets say. Or for the higher end FPGA board, double that speed. Keep in
mind that each hub counts as 1 off the max of 127 devices a single host controller can handle.
Good point, that's a design flaw in the current iteration. The cards would need a redesign to use their own bus so that multiple cards could be daisy chained via a single USB connection. Or use other ways to improve the density of FPGAs per USB port.
Then again, would be cheaper to go mini-itx with a large number of USB ports. A $150 27 watt device with 6 usb ports to power 600 boards (read: over 100 ghash, $360k farm at retail 1 unit pricing) is probably cheaper than any other custom solution. The routers should suffice for up to 20 ghash ($60k farm at retail 1 unit pricing).
The gating factor per power supply would be total wattage, not the number of molex connectors. Each typical consumer $50 500 watt PSUs should be able to power 50 watt FPGAs assuming total draw of 7.5 watts per socket, some cross-loading on 5v and 3.3 and the majority of draw on the 12v. Let's call it another $2/board for power supplies -- so another $1400-2000 in PSUs for the $360k 100 ghash farm.
Either way, the marginal cost over and above the FPGAs themselves is still noise. Even if I suspect the actual boards would be 1/2 to 1/3 the current retail pricing in volume and you're looking at $2000 in ancillary paraphernalia for a $100k 100 ghash installation.