Reading this thread it looks like I way undercharge. I usually tell my buddy to bring his bike and components over with a six pack of beers and we’ll make a Saturday of it. 
$300 though sounds reasonable for a pro to do it all. When you factor in shop space/rent, tools, all other overhead, etc. and the fact that if something is a little off it takes time to redo and get it right, it’s pretty good. I personally though enjoy it so even if it was $20 I’d still do everything myself.