I have my doubts that this device could be sold for much less than the $60
price of the polaroid unit once "real" costs are taken into account (ie no
more getting your transducers from surplus outlets.)  Still, I wouldn't look
a circuit, PCB, and SW working design in the mouth - sounds like lots of fun,
and sure to be at least publishable in one of the hobby magazines (or maybe
it would win Microchips 12C50x design contest.)

BillW