I'd recommend that this is not done using Arduinos - it'd be more power efficient, and better MH/$ to use PIC32MX microcontrollers.
(Although it can be damn hard to get a toolchain set up on Linux because they only released the source for what they have to release... )

Assuming it takes 1984 cycles to do a single SHA256(which it should), you'd get (80MHz/(1984*2)) = (80 * 100 000 000) / 3968 = 2 016 129.03 hashes per second. (I bet I've screwed something up there, somehow )
Of course, I've only included the time for the two hashes. (which is wrong, because you can optimise away part of the last hash, if my beliefs are correct, anyway?), and none of the checking/control stuff.
In this case, I still think you'd only get ~50MH/s for $200.
(THIS IS NOT AT BULK PRICING THOUGH! If you were to buy say, a thousand of them, you might find it to be very different )


And that's at a maximum cost of 8$ + support components. (decoupling capacitors, PCBs, etc)(quite probably less, I can't be bothered to check )

An Arduino would take more clock cycles to do it, I don't know how many, because it's not 32Bit. And it runs more slowly too, so it's unlikely to get you as much money in return.