Here's my stab at the numbers:
I've used this:
to come to the specs on the SoC: A Broadcom BCM2835
From the Broadcom website:
we have access to the "low power ARM1176JZ-F Applications Processor".
I'd like to leave the real meat of the SoC out: the dual core VideoCore IV Multimedia Co-Processor
According to this website:
The ARM1176JZ(F)-S with a variable, MMU + TrustZone Cache will provide, typically 965DMIPS@772 MHz.
Please poke holes in this if you'd like. I didn't realize the core was operating at 772 Mhz. Perhaps it will not. If that number drops to 700MHz, the linear performance would drop to 965*(700/772)=875DMIPS
Compare that to: (from http://en.wikipedia.org/wiki/M.....per_second
An AMD Phenom II X4 940 which is good for 4,282 DMIPS @ 3.0GHz.
4282 DMIPS / 875 DMIPS = 4.9x the performance... by the processor.
If I simplify, the AMD product, costing about $120 (check your local retailer), is about 3.5x the price (no motherboard, no RAM). Add RAM and a MB, and you should break through the MIPS/$ barrier: Raspi wins.
If I look at watts, the 140W(!) used by the AMD is two orders of magnitude in excess of that used by the Raspi. The Raspi will always win here, unless the goal is to boil water.
Now where my gut feeling comes into play:
How does the cluster pass data?
Via that 100baseT connection? Wow, we found our bottleneck, and it's skinny.
Even looking at using USB2.0 you'll never go beyond the 480Mb/s transfer rate. Compare that with Gigabit ethernet for the more conventional system, and you'll need 3-10 Raspis just to match that bandwidth.
This is where the case for a Raspi cluster with $35 per point upgrade starts to fall apart.
Am I making a reasoned and at least partially correct back of envelope analysis here?
I'm coming to believe that the Raspi may make a cheap cluster testing tool, but as a production device, it doesn't look to be built for the task.