$476 to assemble an 8 3/4 diff

yeah but you also have to know what you are doing with pinion depth and backlash etc.
You are correct, and getting those two items right can involve some trial and error. But it ain’t brain surgery either.
Lots of videos on you tube, this forum has lots of knowledgeable and help full folks. I bet everyone here at FABO that knows exactly how to set up an 8.75 started on that knowledge path by actually building one and not by paying someone else to build it.