Some of the LPCs can be had for 60 cents for 1K chip count. That's a 75MHz ARM core instead of a 20MHz AVR core... So a much lower cost per core, a much higher performance per core. I'm a fan of AVRs, but still, there's such a thing as trying too hard -- which is why I laugh at everyone still trying to do "real work" on their CP/M system.
Let us read from the Big Book
of things and witness it's power....
- ARM Cortex-M3 processor, running at frequencies of up to 120 MHz on high speed versions (LPC1769 and LPC1759), up to 100 MHz on other versions, around 100 MIPS?
- Price around 7.5$ for large quantities for most package forms
- no power usage details, possibly less than100mW
- requires board components for development/implementation worth around 10-20$?
So it looks like 100Khash/s for 10$/core using 300mW. Maybe you can buy them cheaper.