I dunno about the NES but if I recall correctly it was used as a low cost Dataglove for PC applications (modded to use PC serial port), the NES one is basically a simplified version of the commercial Dataglove with less precision on finger bending and some other stuff. Of course since the NES has ZERO 3D support at all, and with only one or two games (the rest are just standard NES pad emulation) the power glove is "shit".
Why is it expensive? I guess that's just COLLECTARDS ! ! !