I remember making something similar to this about 20 years ago using a video camera viewfinder (a tiny Cathode ray tube)
http://www.threehillsobservatory.co.uk/astro/UC15_viewfinder.htm
to use on the scope with my long exposure modified video camera
http://www.threehillsobservatory.co.uk/astro/1004xcam1.htm
When you compare with what you get with smart scopes these days and the cost of displays like in VR headsets for example it does seem rather expensive though. If they are popular enough, I can imagine someone undercutting them substantially.