It is certainly not cheap, together with the SD cage I payed the same as the camera cost. However, the hero4 black I have delivers much better video than my Oly E-PL5, that's why I went gopro for videos. The system cost is still quite cheaper than e.g an LX100. Later I might put 2 cams together for a 3D system - this is a mission impossible with the larger cameras because of the large interaxial.
The G140 is a very expensive of port, you must be a very dedicated goPro user to invest in that kind of accessories. The 14-42mm PZ can accept wet wide angle lens and up to 110 degree. For the price of goPro and the G140, doesn't it make more sense to pay a bit more to get a 8mm lens instead and it's more future proof?