Sorry, I do not see your point that power over capital investment cost changes with difficult. Can you please ellaborate or give a practical example ?
Today, you buy $2000 worth of hardware. Your hash rate is 1500 MH/s. You draw 1200 watts. A bitcoin is worth $7. Electricity is $0.10/kWh. You gross $1228 a month and your electricity cost is $86 a month. Your electricity cost is just noise.
Imagine that a year from now, the difficulty is
tenfold.
At this point a FPGA rig is going to have a fraction of your power consumption. At that point:
Your hash rate is 1500 MH/s. You draw 1200 watts. You gross $123 a month and your electricity cost is $86 a month. Your net profit is $37/month. Your electricity cost consumes most of your gross.
The guy with the FPGA array has the same hash rate of 1500 MH/s. He draws 300 watts. He grosses $123 a month and his electricity cost is $21 a month. His net profit is $102 a month.
Will difficulty rise tenfold? Will bitcoins continue to be worth $7? Will 1500MH/s of hashing power only draw 300 watts in FPGAs? Who knows? I'm just pointing out the scenario where the FPGA guy can have a huge advantage.
In your example, you are overriding the initial FPGA costs. At this moment, for a 1500 Mhps FPGA array, you need 18.7 x 80 Mhps FPGA, or $ 11100.- Same 1500 Mhps is achieved with $2000 worth GPU's. So, assuming a profit gain for FPGA's of 102-37 = 65 a month, you still need almost 12 years to recover initial FPGA investment.