you have to amplify electrical signals from the scalp that are in the order of microvolts, which requires having low impedance between the electrode and scalp,

you have to amplify electrical signals from the scalp that are in the order of microvolts, which requires having low impedance between the electrode and scalp, proper amplification circuitry, and the ability to filter out lots of environmental electrical noise. I am a student in a lab where we have a EGI system that costs in the ballpark of ~$100k (with $10-20k in nets that need to be replaced every couple of years), and it is even a world of difference between that system and a $30k mobile EEG system that we recently acquired.