In the previous HN thread on this topic[1], a HN member pointed out that he had built a device for doing this for around $30.00.
If anything, the bigger problem than the price to build the device would probably be the regulatory stuff. I mean, you just know that the government will immediately declare something like this to be a "medical device" and insist on an elaborate, byzantine, corrupt, kafkaesque, bureaucratic nightmare of a regulatory process to get something like this approved.
I can't help but imagine all the cranks who'd sell smartness treatments and do permanent damage to the brain... homeopathy but with a chance of actual damage.