$476 to assemble an 8 3/4 diff

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.
I did ford 8.8s at the dealership, I agree not brain surgery. That's easy to say but they are easy to mess up also, I've seen young techs screw up tires, brake jobs, typical beginner stuff. I once saw an idiot drain the trans and overfill the oil on a Subaru. Another idiot says to me "I forgot to put oil in that last car I had in" I say "well go put oil it it, he says "I cant there it goes..." and we watch it drive down the street. I said "It will be back inside a half hour...' :)