How/who is a pro at differential rebuilds/ repair

What is your question?

I am having a possible differential problem? I have asked a couple of questions of shops in the area. And found that all that I have asked, seem to be able to find repair parts double /triple what I can find with shorter warrantees on the parts. I'm just curious why such discrepancy in part prices and is there a normal price for this repair with a bearing kit + labor