Intel Xeon Phi 7120P Coprocessor

With x86, adding cores is expensive because each one is exceptionally complex

Not all that expensive... look at the Xeon Phi for example; it's not a main CPU, but it does contain 61 (no idea why it' not 64...) Pentium-level x86 cores with 16GB of RAM: http://www.amazon.com/Intel-Xeon-Phi-7120P-Coprocessor/dp/B0...

but RISC can be simpler, use fewer transistors and operate at lower wattage

Fewer transistors in the core is offset by the need for more cache due to lower instruction density, and especially in a system with many cores, cache misses can be far more costly due to all the cores needing to be fed with code and data. Also, that Xeon Phi mentioned above has only 5B transistors with 61 cores whereas the 32-core Oracle chip has more than double that.

Huge numbers of cores can also be exceptionally power efficient if each core can run in a low power mode until needed

This is true, but the caches also consume a significant amount of power too - Intel's mobile CPUs have had dynamic cache sizing for several years now, where parts of the cache can be turned off to save power.

https://www.altera.com/content/dam/altera-www/global/en_US/p...

"We show that our implementation achieves 5.5x and 5.25x better performance per watt ratios than GPU and CPU implementations, respectively."

This is from Altera and they probably skewed to FPGA side. But still, FPGA can be very competitive if you consider power budget.

Xeon Phi costs $3900: http://www.amazon.com/Intel-Xeon-Phi-7120P-Coprocessor/dp/B0... It consumes up to 300W.

The average kWh in USA is $0.12: http://www.npr.org/sections/money/2011/10/27/141766341/the-p...

So Xeon Phi consumes up to $36 of electricity per hour, $864 per day and electricity start to dominate as quick as in 5 days.

Typical FPGA consume about 10 times as less power as Xeon Phi.