Texas Instruments is coming out with a microchip that takes care of
most of the difficulty with building EEG machines.
Each should cost around $25 for an 8 lead chip. I'm considering buying
a bunch when they are released.
With this chip it looks like the most difficult part to building an
EEG machine is feeding 8 digital signals to a computer.
Or more than 8 if you daisy chain them. There's a formula in the spec
for how many can be daisy chained.