When a neuron fires, an action potential of about 100 mV is generated. An electrode measuring brain activity will record several hundreds neurons at a time. How much voltage an electrode directly in contact with brain tissue is expected to receive?
The Utah array is a multi-electrode array for intracortical recordings. In one of many publications on this device, the authors show some sample traces that reveal amplitudes generally between 5 and 40 microVolts. However, measured voltage will depend on the vigilance state of the brain, recording area, time on the millisecond scale, time on the macro scale (gliosis) and insertion depth and so forth.
Note that extracellular electrodes will never measure the high potentials that occur at the membrane surface as recorded through intracellular electrodes. Because of the distance of extracellular electrodes to the neuronal cell membranes, the signals are much smaller.